off-target effect

An off-target effect is what happens when a drug bumps into proteins it was never meant to touch. A molecule shaped to fit one binding site may, by coincidence of shape and chemistry, also fit loosely into other proteins around the body — and those accidental interactions can cause effects of their own, usually unwanted.

These effects are the mirror image of on-target ones: they arise from the molecule, not from the intended biology, so they tend to be specific to that particular compound and can often be engineered away. By contrast, on-target effects appear across all drugs hitting the same target. A central goal of medicinal chemistry is to widen the gap between the concentration that hits the real target and the concentration at which off-target binding starts — that gap is the selectivity window.

Off-target activity is not always bad. Sometimes it is harmless because the off-target is only engaged at concentrations far above the therapeutic dose; sometimes it is even exploited, when an unexpected off-target effect turns into a new use for an old drug (the basis of much drug repurposing). But several classic safety problems — for example a molecule blocking the heart's hERG potassium channel — are off-target liabilities that chemists routinely screen against and design out early.