The rectal route, though less predictable, can be used when oral ingestion is precluded because the patient is unconscious or when vomiting is present. Approximately 50% of the drug that is absorbed from the rectum will bypass the liver, thus reducing the hepatic first-pass effect.
Factors relevant to absorption are circumvented by intravenous injection of drugs because bioavailability is rapid and complete. Also, drug delivery is controlled, can be adjusted to the response of the patient and is achieved with an accuracy and immediacy not possible by any other procedure. Irritating solutions can be given only in this manner because the drug, if injected slowly, is greatly diluted by the blood. Occasionally, a drug is injected directly into an artery to localize its effect. Diagnostic agents sometimes are administered by this route (e.g., technetium-labeled human serum albumin).
Unfavorable reactions can occur when transiently high concentrations of a drug or its vehicle are attained rapidly in plasma and tissues. There are therapeutic circumstances where it is advisable to administer a drug by bolus injection (e.g., tissue plasminogen activator) and other circumstances where slower administration of drug is advisable (e.g., antibiotics).
Injection of a drug into a subcutaneous site can be used only for drugs that are not irritating to tissue; otherwise, severe pain, necrosis, and tissue sloughing may occur. The rate of absorption following subcutaneous injection of a drug often is sufficiently constant and slow to provide a prolonged effect. Moreover, altering the period over which a drug is absorbed may be varied intentionally, as is accomplished with insulin for injection using particle size, protein complexation, and pH. Absorption of drugs implanted under the skin in a solid pellet form occurs slowly over a period of weeks or months; some hormones (e.g., contraceptives) are administered effectively in this manner.
Drugs in aqueous solution are absorbed rapidly after intramuscular injection depending on the rate of blood flow to the injection site and the fat versus muscular composition of the site. This may be modulated to some extent by local heating, massage, or exercise. Generally, the rate of absorption following injection of an aqueous preparation into the deltoid or vastus lateralis is faster than when the injection is made into the gluteus maximus. The rate is particularly slower for females after injection into the gluteus maximus. Slow, constant absorption from the intramuscular site results if the drug is injected in solution, oil, or various other repository (depot) vehicles.
The blood—brain barrier and the blood-cerebrospinal fluid (CSF) barrier often preclude or slow the entrance of drugs into the CNS. Therefore, when local and rapid effects on the meninges or cere-brospinal axis are desired, drugs sometimes are injected directly into the spinal subarachnoid space. Brain tumors may be treated by direct intraventricular drug administration.
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