Coming from the Latin adversus meaning "turned against," adverse is an adjective describing a factor that seems to work against or actively harm something. Think of the related word, adversary, which means "enemy or opponent," so that if something is adverse, it acts as if it were the enemy.

(AD-vers eh-FEKT) An undesired effect of a drug or other type of treatment, such as surgery. Adverse effects can range from mild to severe and can be life-threatening. Also called adverse event and adverse reaction.

Adverse, usually applied to things, often means "harmful" or "unfavorable" and is used in instances like "adverse effects from the medication." Averse usually applies to people and means "having a feeling of distaste or dislike." It is often used with to or from to describe someone having an aversion to something ...

What is an adverse reaction? Adverse reactions are harmful effects suspected to be caused by a medicine or vaccine. They are also known as adverse drug reactions (ADRs), side effects or undesirable effects. All medicines and vaccines can cause adverse reactions in some people.