When taken simvastatin induce sever headache and dr change to atrovastatin so now ok but my friend taken simvastatin and good with this. (why?)

Adverse reactions: Different people's bodies respond differently to medication. While rare, headache was a reported adverse effect of simvastatin. In most cases of minor (not in intensity, but in likelihood of true allergic response or systemic damage) adverse effects, changing to a different member in the class like your doctor did often solves the issue.