Side effects are unintended symptoms after taking a drug medication regimen. An adverse effect includes unexpected, harmful, or unusual outcomes. Effects range from minor inconveniences to severe adverse events such as liver damage, anaphylaxis, or heart attacks. The most common side effects include stomach upsets, drowsiness, diarrhea, constipation, and a dry mouth.

Based on clinical research, some side effects are expected, as any substance can result in a side effect, even a placebo. Medications like aspirin to prescription drugs, over-the-counter (OTC) drugs, vitamin supplements, alternate and complementary therapies have a risk versus benefit profile.

Side effects also vary depending on patient factors such as age, gender, ethnicity, weight, disease state, and general health of the individual. If a medication leads to hospitalization, disability,  permanent damage, or congenital disability due to exposure during pregnancy, it is considered a severe side effect or Adverse Drug Event (ADE).

Antibiotics are the main therapeutic classification known to cause allergic reactions like skin rash, hives, and anaphylaxis.

Side effects commonly occur when starting, ending, decreasing, and increasing the dosage. A health care provider may adjust the dosage or prescribe a second medication to counter the side effect if they occur. In addition, to minimize a side effect, the individual may be advised to modify their diet or lifestyle.