What Are Drugs?

Drugs are chemical or biological substances that, when introduced into or applied to a living organism, produce a physiological effect by interacting with molecular targets such as receptors, enzymes, ion channels, or nucleic acids. In the pharmaceutical and biomedical context, the term refers to compounds intended to prevent, diagnose, treat, or cure disease, as well as those used to modify normal physiology in a controlled way. The study of drugs spans biochemistry, molecular biology, pharmacology, and clinical medicine, and it is deeply intertwined with the engineering disciplines of drug design, formulation science, and medical device development.

Drugs act through two broad types of molecular interaction. Small-molecule drugs, which have molecular weights typically below 500 daltons, diffuse passively into tissues and bind to specific protein targets. Biological drugs, or biologics, include monoclonal antibodies, therapeutic proteins, vaccines, and nucleic acid therapeutics; these are larger, structurally complex molecules produced by living cells or synthesised biochemically and often interact with extracellular targets or cellular receptors. The distinction shapes how each class is manufactured, formulated, and regulated.

Pharmacokinetics and Pharmacodynamics

Pharmacokinetics describes what the body does to a drug: absorption from the site of administration into systemic circulation, distribution across tissues, metabolism primarily in the liver, and excretion through kidneys, bile, or lungs. Together these processes are abbreviated as ADME and determine the plasma concentration profile over time, which must stay within a therapeutic window bounded by a minimum effective concentration and a toxic concentration. Pharmacodynamics describes what the drug does to the body: the molecular mechanisms by which it produces its effects, characterised by receptor binding affinity, intrinsic activity, and the shape of the dose-response relationship. Quantitative pharmacokinetic/pharmacodynamic (PK/PD) modelling links the plasma concentration to the observed pharmacological response and guides dosing regimen design. The chemical basis of these interactions is covered in NIH-supported work on the chemical basis of pharmacology.

Drug Classification and Development

Drugs are classified in several overlapping ways: by therapeutic class (antihypertensive, antibiotic, antineoplastic), by chemical structure (beta-lactam, statin, corticosteroid), by mechanism of action (enzyme inhibitor, receptor agonist, ion channel blocker), and by regulatory category (prescription, over-the-counter, controlled substance). The development pipeline moves from target identification and lead discovery through preclinical testing in cell and animal models, then through phased clinical trials that establish safety, dosing, and efficacy in human populations before regulatory agencies grant marketing approval. Molecular biomarkers play an increasingly important role in this process, serving as measurable indicators that a drug is engaging its intended target and producing the expected biological effect, enabling adaptive trial designs and patient stratification. A survey of recent advances in medicinal chemistry and drug target mechanisms is provided in PMC research on breakthroughs in medicinal chemistry.

Biochemical Foundations

The interaction between a drug and its molecular target is governed by biochemical principles: the three-dimensional complementarity of binding surfaces, non-covalent forces (hydrogen bonds, van der Waals interactions, hydrophobic contacts, and ionic interactions), and in some cases covalent bond formation. Enzyme inhibitors can be competitive, non-competitive, or irreversible, each with distinct kinetic signatures and implications for dosing. G-protein-coupled receptors (GPCRs) are the largest class of druggable targets in the human proteome, and approximately one-third of approved drugs act on them. Research programs at the NIH Intramural Research Program in molecular pharmacology investigate the fundamental mechanisms by which candidate compounds interact with protein targets, providing the biochemical data that guides medicinal chemistry optimisation.

Applications

Drugs have applications in a wide range of fields, including:

• Cardiovascular medicine, for management of hypertension, heart failure, arrhythmia, and dyslipidaemia
• Oncology, through cytotoxic agents, targeted kinase inhibitors, and immune checkpoint biologics
• Infectious disease, via antibiotics, antivirals, antifungals, and antiparasitic agents
• Neurology and psychiatry, for treatment of epilepsy, depression, schizophrenia, and neurodegenerative disease
• Diagnostics and imaging, using radiolabelled drugs and contrast agents to visualise physiological processes