Side effect
Etymology
A transparent modern compound: "side" (off to one side, secondary) plus "effect." It frames the intended result as central and everything else as happening off to the side — a spatial metaphor for unintended consequences.
noun phrase · /SYD ih-fekt/
A secondary effect of a treatment beyond the one intended.
What it names
Use it for any unintended consequence of a drug — which can be neutral, harmful, or occasionally useful.
Example
Drowsiness was a side effect of the allergy pill, which is why it doubled as a sleep aid.
Related idea
Strictly, a side effect is any secondary effect and can even be beneficial, while an adverse effect is specifically a harmful one. Drug labels use "adverse" for the dangers precisely because "side effect" is too neutral to warn.