01.08.1 Intravenous

Injection into a vein (intravenous) relies on the skill of finding a vein, getting a needle into the vein, and keeping the vein open during the injection. This is a skill that relies on practice at the patient’s expense! One of the major advantages of the intravenous (i.v.) route is that it gives more immediacy and accuracy than other routes. Immediacy may be required in an emergency situation. In an emergency situation it may be necessary to sedate the subject quickly. After oral administration of a drug, it takes between 15-30 minutes to start to get any sedation. When the sedative midazolam is used intravenously, sedation occurs in 2-4 minutes.

In a situation where a subject has intense pain, quick pain relief is required. Morphine is a potent analgesic (pain reliever). For quick pain relief, morphine is administered intravenously, and pain relief occurs in 10-15 minutes, whereas if morphine is administered orally, to takes 30 minutes to cause pain relief.

After oral administration, in addition to the time lag for absorption, there may be a reduction in drug activity by liver metabolism, which gives a range of bioavailability, which is specific for each drug. In contrast, intravenous administration avoids first pass liver metabolism, allows for a rapid effect, and the bioavailability is 100%. Because of the greater bioavailability after intravenous administration, a lower dose may be required after intravenous administration than oral administration. For instance, when the pain killer pethidine is administered orally, the dose is 100 mg, and there is extensive first pass liver metabolism. When pethidine is administered intravenously, there is no first pass metabolism, and the dose is 25 mg.

The intravenous route of administration can be used for drugs that are not active after oral administration e.g. ionised drugs, peptides. The intravenous route of administration can often be used for drugs that irritate the intestinal mucosa, as blood vessel walls are quite insensitive. Many of the cytotoxic (cell killing) anti-cancer drugs irritate the intestinal mucosa but not the blood vessels walls, and are given intravenously

One disadvantage of the intravenous route is that it is not suitable for all drugs. Drugs in oily vehicles or drugs that precipitate blood constituents or haemolyse red blood cells cannot be used intravenously. Another disadvantage of the intravenous route is that, with oral administration of the wrong drug or the wrong dose of a drug, there is an opportunity to remove the drug during absorption, and before it reaches the circulation, but this is not possible after intravenously administration. Thus, it is a disadvantage that the actions of an intravenously administered drug are often not easily reversed.