General Principles of Pharmacology

The chemical name specifies the exact molecular structure of a drug using systematic rules. Chemical names are precise but unwieldy for clinical use — the chemical name for acetaminophen is N-(4-hydroxyphenyl)acetamide — and they appear primarily in pharmaceutical chemistry, not at the bedside.

The generic name, also called the nonproprietary name, is the standardized publicly owned name assigned to an active ingredient. The World Health Organization coordinates an international system of generic names called International Nonproprietary Names. These names are organized by drug class using stems: a shared suffix or prefix that signals membership in a pharmacological class. Learning the common stems allows immediate class recognition from a drug name alone, even for an unfamiliar agent. Prescribing by generic name is standard practice in most institutional settings.

The brand name is assigned by the manufacturer and protected by trademark. A single generic drug may have several brand names across different manufacturers. Once the original patent expires, other manufacturers may produce and sell the same active ingredient as a generic product approved on the basis of bioequivalence, meaning it delivers the same amount of drug to the circulation in the same time frame as the reference product.

Drugs are derived from four principal sources, each with characteristic examples that appear throughout pharmacology.

Enteral routes deliver drug via the gastrointestinal tract. The oral route is the most common, most convenient, and most studied route of drug administration. Tablets and capsules dissolve in the stomach and small intestine, where drug molecules are absorbed across the intestinal wall into the portal circulation. Before reaching the systemic bloodstream, orally absorbed drugs pass through the liver, which can metabolize a substantial fraction of the absorbed dose before it ever reaches the target organ. This process is called the first-pass effect, and it reduces the fraction of the administered dose that reaches the systemic circulation — a quantity known as bioavailability. Drugs with extensive first-pass metabolism have low oral bioavailability and may need to be administered by a different route, or at much higher oral doses than they would require intravenously.

The sublingual route, where a tablet dissolves under the tongue, allows drug to be absorbed directly into the venous drainage of the mouth and bypass the liver entirely. Nitroglycerin is the prototypical sublingual drug: its first-pass metabolism is so extensive that it would be therapeutically ineffective given orally at standard doses, but it is highly effective sublingually because the liver does not see it before it reaches the coronary circulation.

Parenteral routes bypass the gastrointestinal tract entirely. Intravenous administration delivers drug directly into the bloodstream, achieving 100 percent bioavailability by definition. This makes intravenous the preferred route when a rapid, predictable, and precisely controllable plasma concentration is required — in emergencies, in critically ill patients, and for drugs so poorly absorbed orally that the gastrointestinal route is not clinically viable.

Intramuscular injection deposits drug into muscle tissue, from which it diffuses into capillaries. Absorption is faster than oral but slower than intravenous, and bioavailability is generally high. Subcutaneous injection deposits drug into the tissue beneath the skin; absorption is slower still, which makes this route suitable for drugs intended to act over hours, such as insulin. Some formulations use this slow absorption deliberately to achieve depot effects, where drug is released gradually over days or weeks from the injection site.

Inhalation delivers drug directly to the pulmonary epithelium. The large surface area and rich blood supply of the lung allow rapid absorption, and topical delivery to the airway maximizes drug concentration at the target while minimizing systemic exposure. This makes inhalation the preferred route for bronchodilators and inhaled corticosteroids used to treat asthma and chronic obstructive pulmonary disease. Transdermal delivery through the skin provides sustained, slow release that avoids first-pass metabolism and maintains relatively constant plasma concentrations. Fentanyl patches for chronic pain and nicotine patches for cessation therapy are familiar examples.

Route selection is one element of the broader rational prescribing framework. Rational prescribing means selecting the right drug, at the right dose, by the right route, for the right duration, in the right patient. Every component of this framework requires pharmacological reasoning. The right drug means choosing based on evidence of efficacy for a specific indication, not habit or familiarity. The right dose means individualizing based on the patient’s weight, renal function, hepatic function, age, and relevant genetic factors rather than applying a universal standard. The right route follows from the clinical urgency and the drug’s absorption properties. The right duration requires knowing how long a drug needs to be given to achieve its therapeutic goal and when continued use becomes net harmful. The right patient means assessing contraindications and considering all other medications the patient is taking, including non-prescription drugs and supplements, for potential interactions.