Which of the following are sources of individual variability in drug response?

• A. Genetic factors, environment, demographic factors, disease, drug interactions, and idiosyncratic factors.
• B. Genetic factors only.
• C. Environmental factors only.
• D. Disease only.

Answer: (C)

Variability in how a person responds to a drug comes from many interacting influences. Genetic makeup can change how quickly a drug is broken down or how strongly it acts at its target, through differences in enzymes, transporters, or receptors. Environmental factors like diet, co-administered substances, and adherence can modify exposure and effect. Demographic traits such as age, sex, body size, and pregnancy can shift pharmacokinetics and pharmacodynamics. The presence of disease, especially hepatic or renal impairment, can alter clearance and distribution. Drug interactions from other medications or foods can induce or inhibit metabolic pathways or transporters, changing levels and responses. Some reactions are idiosyncratic—unpredictable and not explained by dose or common factors—often immune-mediated. Because all these elements contribute, a complete description of sources of variability includes genetic, environmental, demographic, disease, drug interactions, and idiosyncratic factors rather than environmental factors alone.