The parenteral route is a critical method of drug administration. It delivers compounds directly into the systemic circulation and bypasses the gastrointestinal tract. This approach is particularly advantageous for drugs that exhibit poor absorption or instability when administered orally.

There are three primary parenteral routes: intravenous (IV), intramuscular (IM), and subcutaneous (SC). The IV route introduces the drug directly into the bloodstream, ensuring immediate action. The IM route targets muscle tissue, while the SC route reaches beneath the skin. In IM and SC administration, drug molecules diffuse along a gradient toward proximate blood vessels.

In practice, IV injections are commonly connected to a hung bag via a line. Certain IV drugs, such as tissue plasminogen activator, are delivered via bolus injection to secure rapid effects. In contrast, antibiotics are infused slowly over a longer period to ensure prolonged action and to prevent toxicity. IM drugs in aqueous solutions are rapidly absorbed, while specialized depot preparations provide a more gradual release. Notably, the absorption rate differs based on the injection site; for instance, injections into the deltoid muscle absorb faster than those into the gluteus maximus. Subcutaneously administered drugs, including insulin and contraceptive implants, demonstrate slow absorption rates, thereby extending their therapeutic effect. However, it's crucial to note that subcutaneous injection of tissue irritants can induce necrosis and severe pain.

Some additional parenteral routes include intradermal, intra-arterial, intrathecal, and intraventricular administration. Intradermal delivery involves administering drugs or vaccines into the dermis, the skin layer just below the epidermis, allowing localized drug absorption. It's commonly used for allergy tests, vaccines like BCG, and some cosmetic treatments like botox injections. Intra-arterial drug administration is occasionally used to target specific tissues or organs, such as liver tumors and head and neck cancers. However, accidental intra-arterial administration can lead to severe complications. Intrathecal delivery involves injecting drugs into the subarachnoid space to reach the brain. It bypasses the blood-brain and blood-cerebrospinal fluid (CSF) barriers, which often restrict drug entry into the central nervous system (CNS). This method is beneficial when rapid effects on the meninges or spinal axis are required, as in spinal anesthesia. For instance, baclofen injection effectively alleviates severe muscle spasms. Direct intraventricular drug administration treats brain tumors or severe CNS infections, often via long-term indwelling reservoir devices.