Intravenous therapy or IV therapy is the giving of liquid substances directly into a vein. It can be intermittent or continuous; continuous administration is called an intravenous drip. The word intravenous simply means "within a vein", but is most commonly used to refer to IV therapy. Therapies administered intravenously are often called specialty pharmaceuticals.

Compared with other routes of administration, the intravenous route is the fastest way to deliver fluids and medications throughout the bodyFact: date=September 2008. Some medications, as well as blood transfusions and lethal injections, can only be given intravenously.

Needle and syringe

The simplest form of intravenous access is a syringe with an attached hollow needle. The needle is inserted through the skin into a vein, and the contents of the syringe are injected through the needle into the bloodstream. This is most easily done with an arm vein, especially one of the metacarpal veins. Usually it is necessary to use a tourniquet first to make 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; then the tourniquet is removed before injecting.

This is the most common method of intravenous drug use for euphoriants such as heroin, or in any case where a person must self-administer intravenous medication at home. It is also a convenient way to deliver life-saving medications in an emergency. However, in a controlled health-care setting, direct injection is rarely used since it only allows delivery of a single dose of medication.

Peripheral IV lines

This is the most common intravenous access method in both hospitals and pre-hospital services. A peripheral IV line consists of a short catheter (a few centimeters long) inserted through the skin into a peripheral vein, any vein that is not in the chest or abdomen. There are times, however, when underlying physiological factors (morbid obesity, peripheral vascular disease and IV drug abuse, to name a few) make insertion into any available vein a medical necessity--particularly if the patient is exsanguinating. The adage "time is tissue" should be paramount during times like these or if the patient is at risk for a cardiac event. Arm and hand veins are typically used although leg and foot veins are occasionally used. Veins in the arm are the common site in emergency settings, commonly performed by paramedics and emergency physicians. On infants the scalp veins are sometimes used. 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 bung between treatments. Ported cannulae have an injection port on the top that is often used to administer medicine. The caliber of cannula is commonly indicated in gauge, with 14 being a very large cannula (used in resuscitation settings) and 24-26 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 actually deliver equivalent volumes of fluid faster than central lines, accounting for their popularity in emergency medicine; these lines are frequently called "large bores" or "trauma lines".