An adverse reaction is a negative reaction to a medical procedure or medication. Also known as a negative side effect, these can be caused by a physical sensitivity or allergy to a medication. Other causes can include taking medications too often, not often enough, or in the wrong dosage. Adverse reactions can also be caused by starting or stopping a medication which may cause complications and negatively affect the prognosis. Some medications have well-known side effects such as weight gain, gastric problems (constipation, diarrhea, upset stomach), or mood changes. Adverse reactions and side effects are required to be listed in the documentation that comes with medication.