The intravenous route of administration is commonly used for rehydration or to provide nutrients for those who cannot, or will not—due to reduced mental states or otherwise—consume food or water by mouth.

Attempts at providing intravenous therapy have been recorded as early as the 1400s, but the practice did not become widespread until the 1900s after the development of techniques for safe, effective use.

The intravenous route is the fastest way to deliver medications and fluid replacement throughout the body as they are introduced directly into the circulatory system and thus quickly distributed.

The most basic intravenous access consists of a needle piercing the skin and entering a vein which is connected to a syringe or to external tubing.

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

Volume expansion consists of the administration of fluid-based solutions or suspensions designed to target specific areas of the body which need more water.

For this reason certain types of medications can only be given intravenously, as there is insufficient uptake by other routes of administration,[9] such is the case of severe dehydration where the patient is required to be treated via IV therapy for a quick recovery.

[11] Oral medications also may be less desirable if a person is nauseous or vomiting, or has severe diarrhea, as these may prevent the medicine from being fully absorbed from the gastrointestinal tract.

A person receiving parenteral nutrition will be given an intravenous solution which may contain salts, dextrose, amino acids, lipids and vitamins.

[17] The specific imaging technique being employed will determine the characteristics of an appropriate contrast agent to increase visibility of blood vessels or other features.

[19] Players suspended after attending "boutique IV clinics" which offer this sort of treatment include footballer Samir Nasri in 2017[20] and swimmer Ryan Lochte in 2018.

[22] 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.

[22] Intravenous therapy is also used in people with acute ethanol toxicity to correct electrolyte and vitamin deficiencies which arise from alcohol consumption.

[24] Intravenous access is also sometimes used outside of a medical setting for the self-administration of recreational drugs, such as heroin and fentanyl, cocaine, methamphetamine, DMT, and others.

Certain medications, such as potassium, are not able to be administered by IV push due to the extremely rapid onset of action and high level of effects.

[33] A peripheral cannula is the most common intravenous access method utilized in hospitals, pre-hospital care, and outpatient medicine.

12- and 14-gauge peripheral lines are capable of delivering large volumes of fluid very fast, accounting for their popularity in emergency medicine.

Using a tunneled line reduces the risk of infection as compared to other forms of access, as bacteria from the skin surface are not able to travel directly into the vein.

A tunnelled line is an option for long term venous access necessary for hemodialysis in people with poor kidney function.

[32]: 316–321, 344–348 To reduce pain associated with the procedure, 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.

[44] The incidence of side effects specific to a medication can be affected by the type of access (peripheral versus central), the rate of administration, or the quantity of drug administered.

A peripheral IV line cannot be left in the vein indefinitely out of concern for the risk of infection and phlebitis, among other potential complications.

[47] Phlebitis is particularly common in intravenous drug users,[48] and those undergoing chemotherapy,[49] whose veins can become sclerotic and difficult to access over time, sometimes forming a hard, painful "venous cord".

[50] Infiltration occurs when a non-vesicant IV fluid or medication enters the surrounding tissue as opposed to the desired vein.

As stated by Wren, "I Have Injected Wine and Ale in a liveing Dog into the Mass of Blood by a Veine, in good Quantities, till I have made him extremely drunk, but soon after he Pisseth it out."

It was difficult to find people who would agree to be transfused, but an eccentric scholar, Arthur Coga, consented and the procedure was carried out by Lower and King before the Royal Society on 23 November 1667.

There was virtually no recorded success with any attempts at injection therapy until the 1800s, when in 1831 Thomas Latta studied the use of IV fluid replacements for cholera treatment.

[56] In the 1830s, James Blundell, an English obstetrician, used intravenous administration of blood to treat women bleeding profusely during or after delivery.

Intravenous therapy was expanded by Italian physician Guido Baccelli in the late 1890s[62] and further developed in the 1930s by Samuel Hirschfeld, Harold T. Hyman and Justine Johnstone Wanger[63][64] but was not widely available until the 1950s.

[65] There was a time, roughly the 1910s–1920s, when fluid replacement that today would be done intravenously was likelier to be done with a Murphy drip, a rectal infusion; and IV therapy took years to increasingly displace that route.