If you want to describe a negative reaction to something (such as a harmful side effect from medication) or dangerous meteorological conditions (such as a snowstorm), adverse is the correct choice; you would not say that you had an averse reaction to medication or that there was averse weather.

Adverse refers to unfavorable, harmful, or detrimental conditions or effects that are not desirable or expected; contradictory, opposing in position or direction.

Adverse Condition means a condition which is producing, or which has the potential to produce, a detrimental effect on a system or component that either impairs the system or component’s normally intended function or operation or which is inconsistent with generally established practice (s) regarding the historically or conventionally applied and acknowledged methods of installation, assembly, and operation or use;