Choosing the appropriate route of drug administration is significantly influenced by two key factors: the therapeutic objectives and the inherent properties of the drug being used.

Administering drugs via inhalation allows for the direct delivery of gaseous, volatile substances or droplets to different parts of the respiratory tract. One of the advantages of the inhalation route is the rapid absorption of drugs into the circulatory system, which is possible because of the large surface area of the lungs. This large surface area facilitates efficient absorption of inhaled medications, leading to localized effects and fewer systemic side effects. However, controlling the dosage of inhaled drugs can be tricky, and these medications may trigger a cough reflex, causing patient discomfort.

The intrathecal route delivers drugs directly into the cerebrospinal fluid, providing immediate and localized effects, making it highly effective in specific clinical scenarios. For instance, when the same drug is administered orally, the blood-brain barrier may hinder or delay its absorption into the central nervous system.

In topical applications, drugs are directly applied to the mucous membranes of the affected parts to exert their therapeutic effects locally.

Transdermal absorption offers systemic effects, and the rate at which a drug is absorbed depends on various factors. The advantage of this route is its ability to maintain consistent blood drug concentration, thereby reducing toxicity risk.

Lastly, the rectal route of administration is often preferred when a patient is unconscious or vomiting. This route allows the drug to bypass hepatic metabolism and prevents the drug from being destroyed in the stomach.