Intravenous therapy (IV) is a therapy that delivers fluids directly into a vein. The intravenous route of administration can be used both for injections, using a syringe at higher pressures; as well as for infusions, typically using only the pressure supplied by gravity. Intravenous infusions are commonly referred to as drips.

The intravenous route is the fastest way to deliver medications and fluid replacement throughout the body, because they are introduced directly into the circulation. Intravenous therapy may be used for fluid volume replacement, to correct electrolyte imbalances, to deliver medications, and for blood transfusions.

Intravenous systems can be categorized by which type of vein the inserted tube, called the catheter, empties into.

A peripheral intravenous (PIV) line is used on peripheral veins (the veins in the arms, hands, legs and feet). This is the most common type of IV therapy used.

Central IV lines have their catheters that are advanced through a vein and empty into a large central vein (a vein within the torso), usually the superior vena cava, inferior vena cava or even the right atrium of the heart.

Indications for a central line over the more common peripheral IV line commonly includes poor peripheral venous access for a PIV. Another common indication is when patients would require infusions over a prolonged period of time, such as antibiotic therapy over a few weeks for osteomyelitis. Another indication is when the substances to be administered could irritate the blood vessel lining such as total parenteral nutrition, whose high glucose content can damage blood vessels, and some chemotherapy regimens. There is less damage to the blood vessels because central veins have a larger diameter than peripheral veins, have faster blood flow, and would get diluted as it is quickly distributed to the rest of the body. Vasopressors (such as norepinephrine, vasopressin, epinephrine, phenylephrine, among others) are typically infused through central lines to minimize the risk of extravasation.

Other advantages are that multiple medications can be delivered at once, even if they would not be chemically compatible within a single tube as there is room for multiple parallel compartments (lumina) within the catheter. It is commonly believed that fluid can be pushed faster through a central line; however, the diameter of each lumen is often smaller than that of a large-bore peripheral cannula. Caregivers can also measure central venous pressure and other physiological variables through the central line. They are also longer and, as reflected by Poiseuille's law, require higher pressure to achieve the same flow, all other variables being equal.

Central IV lines carry risks of bleeding, infection, gangrene, thromboembolism and gas embolism (see Risks below). They are often more difficult to insert correctly as the veins are not usually palpable and rely on an experienced clinician knowing the appropriate landmarks and/or using an ultrasound probe to safely locate and enter the vein. Surrounding structures such as the pleura and carotid artery are also at risk of damage with the potential for pneumothorax or even cannulation of the artery.

There are several types of central IV access, depending on the route that the catheter takes from the outside of the body to the vein.

The PICC line is inserted through a sheath into a peripheral vein sometimes using the Seldinger technique or modified Seldinger technique, under ultrasound guidance, usually in the arm, and then carefully advanced upward until the catheter is in the superior vena cava or the right atrium. This is usually done by measuring the distance to an external landmark, such as the suprasternal notch, to estimate the optimal length. An X-ray must be used to verify that the tip is in the right place when fluoroscopy was not used during the insertion. More modern technology utilizes EKG technology to determine when the tip is in the correct location.

A PICC may have a single (single-lumen) tube and connector, two (double-lumen) or three (triple-lumen) compartments, each with its own external connector. Power-injectable PICCs are now available as well. From the outside, a single-lumen PICC resembles a peripheral IV line, except that the tubing is slightly wider.

The insertion site requires better protection than that of a peripheral IV line, due to the higher risk of serious infection if bacteria travel up the catheter. However, a PICC poses less of a systemic infection risk than other central IV lines, because the insertion site is usually cooler and drier than the sites typically used for other central lines. This helps to slow the growth of bacteria which could reach the bloodstream by traveling under the skin along the outside of the catheter.

The chief advantage of a PICC over other types of central lines is that it is safer to insert with a relatively low risk of uncontrollable bleeding and essentially no risks of damage to the lungs or major blood vessels. Although special training is required, a PICC does not require the skill level of a physician or surgeon. It is also externally unobtrusive, and with proper hygiene and care can be left in place for months to years if needed for patients who require extended treatment.

The chief disadvantage is that it must be inserted and then travel through a relatively small peripheral vein which can take a less predictable course on the way to the superior vena cava and is therefore somewhat more time consuming and more technically difficult to place in some patients. As a PICC travels through the axilla, it can also become kinked, causing poor function.

While some central lines have their catheter pass through the skin and then directly into the vein, other central lines called "tunneled catheters" insert through the skin and then pass or "tunnel" a significant distance before inserting into the vein. This reduces the risk of infection, since bacteria from the skin surface are not able to travel directly into the vein. These catheters are often made of materials that resist infection and clotting. These include the Hickman line or Broviac catheter.

A port (often referred to by brand names such as Port-a-Cath or MediPort) is a central venous line that does not have an external connector; instead, it has a small reservoir that is covered with silicone rubber and is implanted under the skin. Medication is administered intermittently by placing a small needle through the skin, piercing the silicone, into the reservoir. When the needle is withdrawn, the reservoir cover reseals itself. The cover can accept hundreds of needle sticks during its lifetime. It is possible to leave the ports in the patient's body for years; if this is done, the port must be accessed monthly and flushed with an anti-coagulant, or the patient risks it getting plugged up. If it is plugged, it becomes a hazard as a thrombus will eventually form with an accompanying risk of embolisation. Removal of a port is usually a simple outpatient procedure; however, installation is more complex and a good implant is fairly dependent on the skill of the radiologist. Ports cause less inconvenience and have a lower risk of infection than PICCs, and are therefore commonly used for patients on long-term intermittent treatment.

A third type is a midline catheter which is inserted into a peripheral vein and advances through the vein, similar to a peripheral IV line, but falls short of emptying into a central vein.

A continuous infusion is primarily used to correct fluid and electrolyte imbalances. This is as opposed to intermittent infusion, when a patient requires medications only at certain times, such as secondary IV and IV push.

The tubing from the bag of fluid being administered that connects directly to the patient is called the primary tubing. Any additional IVs to be administered are connected to the primary tubing and are called secondary IV, or IV piggyback;^[1] this is done instead of placing multiple catheters in the patient. When administering a secondary IV medication, the primary bag is held lower than the secondary bag so that the secondary medication can flow into the primary tubing, rather than fluid from the primary bag flowing into the secondary tubing. The fluid from the primary bag is needed to help flush any remaining medication from the secondary IV from the tubing into the patient.

Some medications are also given by IV "push" or bolus. A syringe containing the medication is connected to an access port in the primary tubing and the medication is administered through the port. The syringe plunger is pressed slowly, if it might irritate the vein or cause a too-rapid effect. Certain medications, such as potassium, are never to be administered by IV push because the spike in medication in the blood from the IV push could be fatal. Once a medicine has been injected into the fluid stream of the IV tubing, there must be some means of ensuring that it gets from the tubing to the patient. Usually this is accomplished by allowing the fluid stream to flow normally and thereby carry the medicine into the bloodstream; however, a second fluid injection is sometimes used, a "flush", following the injection to push the medicine into the bloodstream more quickly.

Substances that may be infused intravenously include volume expanders, blood-based products, blood substitutes, medications and nutrition.

There are two main types of volume expander: crystalloids and colloids. Crystalloids are aqueous solutions of mineral salts or other water-soluble molecules. Colloids contain larger insoluble molecules, such as gelatin. Blood is a colloid.

The best way to determine if a person will benefit from fluids is by doing a passive leg raise followed by measuring the output from the heart.^[4]

Medications may be mixed into the fluids mentioned above. Compared with other routes of administration, such as oral medications, the intravenous route is the fastest way to deliver fluids and medications throughout the body. The bioavailability of the IV medication is 100%, unlike oral medications where much of the medication is lost in digestion before entering circulation. Certain types of medications can only be given intravenously, such as when there is insufficient uptake by other routes of administration such as enterally. Examples include intravenous immunoglobulin and propofol.

A blood product (or blood-based product) is any component of blood which is collected from a donor for use in a blood transfusion. Blood transfusions can be life-saving in some situations, such as massive blood loss due to trauma, or can be used to replace blood lost during surgery. Blood transfusions may also be used to treat a severe anaemia or thrombocytopenia caused by a blood disease. People with hemophilia usually need a replacement of clotting factor, which is a small part of whole blood. People with sickle-cell disease may require frequent blood transfusions. Early blood transfusions consisted of whole blood, but modern medical practice commonly uses only components of the blood, such as fresh frozen plasma or cryoprecipitate.

Blood substitutes (also called 'artificial blood' or 'blood surrogates') are artificial substances aiming to provide an alternative to blood-based products acquired from donors. The main blood substitutes used today are volume expanders such as crystalloids and colloids mentioned above. Also, 'oxygen-carrying substitutes' are emerging.

Buffer solutions are used to correct acidosis or alkalosis. Lactated Ringer's solution also has some buffering effect. A solution more specifically used for buffering purpose is intravenous sodium bicarbonate.

Parenteral nutrition is feeding a person intravenously, bypassing the usual process of eating and digestion. The person receives nutritional formulas containing salts, glucose, amino acids, lipids and added vitamins.

A standard IV infusion set consists of a pre-filled, sterile container (glass bottle, plastic bottle or plastic bag) of fluids with an attachment that allows the fluid to flow one drop at a time, making it easy to see the flow rate (and also reducing air bubbles); a long sterile tube with a clamp to regulate or stop the flow; a connector to attach to the access device; and Y-sets to allow "piggybacking" of another infusion set onto the same line, e.g., adding a dose of antibiotics to a continuous fluid drip.

An infusion pump allows precise control over the flow rate and total amount delivered. The volume to be infused (VTBI) of the mainline IV bag is usually programmed for about 50 milliliters less than the stated volume of that IV bag to avoid letting the IV line or tubing run dry. The VTBI for a secondary bag or piggybag should usually be programmed for 30 to 50 milliliters more than is stated to be in that medication IV bag, to make sure that in addition to the bag being emptied, the entire medication dose is flushed through the IV tubing from the mainline bag. Because of its design, the short, secondary IV line cannot run dry. Thus, the registered nurse programs the IV pump for a 50 milliliter bag of IV antibiotics volume to be infused (VTBI) for at least 80 milliliters. The 100 milliliter bag of antibiotics usually needs a VTBI of about 140 milliliters. In cases where a change in the flow rate would not have serious consequences, or if pumps are not available, the drip is often left to flow simply by placing the bag above the level of the patient and using the clamp to regulate the rate; this is a gravity drip.

The simplest form of intravenous access is by passing a hollow needle through the skin directly into the vein. This needle can be connected directly to a syringe (used either to withdraw blood or deliver its contents into the bloodstream) or may be connected to a length of tubing and thence whichever collection or infusion system is desired.

The most convenient site is often the arm, especially the veins on the back of the hand, or the median cubital vein at the elbow, but any identifiable vein can be used. Often it is necessary to use a tourniquet which restricts the venous drainage of the limb and makes the vein bulge. Once the needle is in place, it is common to draw back slightly on the syringe to aspirate blood, thus verifying that the needle is really in a vein. The tourniquet should be removed before injecting to prevent extravasation of the medication.

Many systems of administration employ a drip chamber, which prevents air from entering the blood stream (air embolism), and allows an estimation of flow rate.

A peripheral cannula is the most common intravenous access method utilized in both hospitals and pre-hospital services. A peripheral IV line (PVC or PIV) consists of a short catheter (a few centimeters long) inserted through the skin into a peripheral vein (any vein not situated in the chest or abdomen). This is usually in the form of a cannula-over-needle device, in which a flexible plastic cannula comes mounted over a metal trocar. Once the tip of the needle and cannula are introduced into the vein via venipuncture, the cannula is advanced inside the vein over the trocar to the appropriate position and secured, the trocar is then withdrawn and discarded. Blood samples may be drawn directly after the initial IV cannula insertion.

Any accessible vein can be used although arm and hand veins are used most commonly, with leg and foot veins used to a much lesser extent. In infants, the scalp veins are sometimes used.

The caliber of needles and catheters can be given in Birmingham gauge or French gauge. A Birmingham gauge of 14 is a very large cannula (used in resuscitation settings) and 24-26 is the smallest. The most common sizes are 16-gauge (midsize line used for blood donation and transfusion), 18- and 20-gauge (all-purpose line for infusions and blood draws), and 22-gauge (all-purpose pediatric line). 12- and 14-gauge peripheral lines are capable of delivering large volumes of fluid very fast, accounting for their popularity in emergency medicine. These lines are frequently called "large bores" or "trauma lines".

To make the procedure more tolerable for children, medical staff may apply a topical local anaesthetic (such as EMLA or Ametop) to the skin of the chosen venipuncture area about 45 minutes beforehand.

The part of the catheter that remains outside the skin is called the connecting hub; it can be connected to a syringe or an intravenous infusion line, or capped with a heplock or saline lock, a needleless connection filled with a small amount of heparin or saline solution to prevent clotting, between uses of the catheter. Ported cannulae have an injection port on the top that is often used to administer medicine.

In cases of shock, a central venous catheter, a peripherally inserted central catheter (PICC), venous cutdown or intraosseous infusion may be necessary.

If the cannula is not sited correctly, or the vein is particularly fragile and ruptures, blood may extravasate into the surrounding tissues, this situation is known as a blown vein or "tissuing". Using this cannula to administer medications causes extravasation of the drug which can lead to edema, causing pain and tissue damage, and even necrosis depending on the medication. The person attempting to obtain the access must find a new access site proximal to the "blown" area to prevent extravasation of medications through the damaged vein. For this reason it is advisable to site the first cannula at the most distal appropriate vein.

If a patient needs frequent venous access, the veins may scar and narrow, making any future access extremely difficult or impossible.

A peripheral IV cannot be left in the vein indefinitely out of concern for the risk of infection and phlebitis, among other potential complications. However, recent studies have found that there is no increased risk of complications in patients whose IVs were replaced only when clinically indicated versus patients whose IVs were replaced routinely.^[6] Thus, it is becoming more common to replace IVs only when clinically indicated. There is no need to replace peripheral IVs more frequently than 72–96 hours as long as the IV was placed aseptically.^[7]

Catheter shearing is a very infrequent complication, but a very real danger. Shearing occurs when part of the catheter is cut by the sharp bevelled edge of the trochar. The sheared section may completely separate from the main body of the catheter, and become free floating in the blood stream. The majority of the time, it is due to poor technique, but infrequently a poorly manufactured catheter may break from the hub or shear. Infection, and a foreign body embolus are the two threats to the patient.

A rapid infuser can be used if the patient requires a high flow rate and the IV access device is of a large enough diameter to accommodate it. This is either an inflatable cuff placed around the fluid bag to force the fluid into the patient or a similar electrical device that may also heat the fluid being infused.

An injection inherently causes pain when the skin is broken and is medically invasive. In cases in which a choice between intravenous therapy and oral treatment may be made to achieve the same outcome, such as in the case of mild or moderate dehydration treatment (assuming oral rehydration therapy is an option), then one should avoid using intravenous therapy in place of the less invasive oral option.^[8] Children in emergency departments being treated for dehydration in particular have better outcomes with oral treatment because it does not cause the pain or risk the complications of an injection.^[8]

Cold spray can decrease the pain of putting in an IV.^[9]

Any break in the skin carries a risk of infection. Although IV insertion is an aseptic procedure, skin-dwelling organisms such as Coagulase-negative staphylococcus or Candida albicans may enter through the insertion site around the catheter, or bacteria may be accidentally introduced inside the catheter from contaminated equipment. Moisture introduced to unprotected IV sites through washing or bathing substantially increases the infection risks.

Infection of IV sites is usually local, causing easily visible swelling, redness, and fever. If bacteria do not remain in one area but spread through the bloodstream, the infection is called sepsis and can be rapid and life-threatening. An infected central IV poses a higher risk of sepsis, as it can deliver bacteria directly into the central circulation.

Phlebitis is inflammation of a vein that may be caused by infection, the mere presence of a foreign body (the IV catheter) or the fluids or medication being given. Symptoms are warmth, swelling, pain, and redness around the vein. The IV device must be removed and if necessary re-inserted into another extremity.

Due to frequent injections and recurring phlebitis, scar tissue can build up along the vein. The peripheral veins of intravenous drug addicts, and of cancer patients undergoing chemotherapy, become sclerotic and difficult to access over time, sometimes forming a hard, painful “venous cord”.

Infiltration occurs when an IV fluid or medication accidentally enters the surrounding tissue rather than the vein. It may occur when the vein itself ruptures (the elderly are particularly prone to fragile veins due to a paucity of supporting tissues), when the vein is damaged during insertion of the intravascular access device, when the device is not sited correctly, from increased vein porosity or when the entry point of the device into the vein becomes the path of least resistance (e.g. if a cannula is in a vein for some time, the vein may scar and close and the only way for fluid to leave is along the outside of the cannula where it enters the vein). Infiltration is an inadvertent administration of a nonvesicant solution/drug into the tissue, which happens so often when the tourniquet isn't removed in a timely fashion. Infiltration is characterized by coolness and pallor to the skin as well as localized swelling or edema. It is treated by removing the intravenous access device and elevating the affected limb so that the collected fluids can drain away. Sometimes injections of hyaluronidase can be used to speed the dispersal of the fluid/drug. Infiltration is one of the most common adverse effects of IV therapy^[10] and is usually not serious unless the infiltrated fluid is a medication damaging to the surrounding tissue, most commonly a vesicant or chemotherapeutic agent, in which case it is called extravasation and extensive necrosis can occur.^[11][12]

This occurs when fluids are given at a higher rate or in a larger volume than the system can absorb or excrete. Possible consequences include hypertension, heart failure, and pulmonary edema.

The human body is at risk of accidentally induced hypothermia when large amounts of cold fluids are infused. Rapid temperature changes in the heart may precipitate ventricular fibrillation.

Administering a too-diluted or too-concentrated solution can disrupt the patient's balance of sodium, potassium, magnesium, chloride, and other electrolytes. Hospital patients usually receive blood tests to monitor these levels. It is essential to correct these imbalances if they occur, as they can lead to the clinical symptoms of electrolyte imbalance, which, if left untreated, can lead to acidosis/alkalosis, and ultimately death.

A blood clot or other solid mass, as well as an air bubble, can be delivered into the circulation through an IV and end up blocking a vessel; this is called embolism. It is nearly impossible to inject air through a peripheral IV at a dangerous rate. The risk is greater with a central IV.

Air bubbles of less than 30 microliters are thought to dissolve into the circulation harmlessly. A larger amount of air, if delivered all at once, can cause life-threatening damage, or, if extremely large (3-8 milliliters per kilogram of body weight), can stop the heart.

One reason veins are preferred over arteries for intravascular administration is because the flow will pass through the lungs before passing through the body. Air bubbles can leave the blood through the lungs. A patient with a right-to-left shunt is vulnerable to embolism from smaller amounts of air. Fatality by air embolism is rare, although this may be in part because it is so difficult to determine when this is the cause of death.

Intravenous glucose is used in some Asian countries such as Korea as a pick-me-up, for "energy," but is not a part of routine medical care in the United States where a glucose solution is a prescription drug. Asian immigrants to the United States are at risk if they seek intravenous glucose treatment. It may be had at store-front clinics catering to Asian immigrants, but, despite having no more effect than drinking sugared water, poses medical risks such as the possibility of infection. It is commonly called "ringer."^[13]

IV rehydration was formerly a common technique for athletes.^[14] The World Anti-Doping Agency (WADA) prohibits intravenous injection of more than 100mL per 12 hours, except under a medical exemption.^[14] The United States Anti-Doping Agency notes that, as well as the dangers inherent in IV therapy, "IVs can be used to change blood test results (such as hematocrit where EPO or blood doping is being used), mask urine test results (by dilution) or by administering prohibited substances in a way that will more quickly be cleared from the body in order to beat an anti-doping test".^[14] Sportspeople suspended after attending boutique IV clinics include footballer Samir Nasri in 2017^[15] and swimmer Ryan Lochte in 2018.^[16]

In the 1960s, John Myers developed the "Myers' cocktail", a non-prescription IV solution of vitamins and minerals marketed as a hangover cure and general wellness remedy.^[17] The first "boutique IV" clinic, offering similar treatments, opened in Tokyo in 2008.^[17] These clinics, whose target market was described by Elle as "health nuts who moonlight as heavy drinkers", have been publicized in the 2010s by glamorous celebrity customers.^[17]

Intravenous technology stems from studies on cholera treatment in 1831 by Dr Thomas Latta of Leith.^[18] Intravenous therapy was pioneered by Italian physician Guido Baccelli in the late 1890s^[19] and further developed in the 1930s by Hirschfeld, Hyman and Wanger^[20][21] but was not widely available until the 1950s.^[22]