Why do some people experience more side effects from medication than others?

People are different: Some people are more susceptable to drugs, etc. Than others, and so have more side effects. This can be because their metabolism for a particular drug is faster, and more extensive. For example, some people are more sensitive to alcohol than others, so they get intoxicated with very little while others need a great deal for it to take effect. Those who often drink can build a tolerance to it.

Liver: Liver which is not healthy can make people hold on to metabolites. Important to have healthy liver to not have problems.