How are most drug side effects produced?

• A. From selective drugs producing off-target effects.
• B. From the body's natural receptor activity.
• C. From non-selective drugs interacting with receptors it shouldn't.
• D. From the drug's metabolism only.

Answer: (C)

Most side effects come from the drug engaging receptors it shouldn’t touch. When a drug isn’t perfectly selective, it can bind to multiple receptor types or subtypes in different tissues. That off-target binding alters normal physiology in places where you don’t want an effect, producing unwanted symptoms. The same drug that relieves one problem can cause others if it activates or blocks receptors in other organs, leading to dizziness, dry mouth, constipation, drowsiness, or even bronchoconstriction in susceptible individuals.

Even drugs labeled as selective can produce these effects if they have enough affinity for other receptors at therapeutic doses or accumulate in tissues with those receptors. While metabolism and active metabolites can contribute to side effects, the primary driver is pharmacodynamic off-target receptor interaction—binding where it isn’t intended.

So, most adverse effects reflect the body’s different receptor landscapes and how broad a drug’s receptor activity is, rather than metabolism alone.