Every drug has effects beyond the one it was prescribed for. A side effect is any unintended consequence of a medication, whether harmful, neutral, or even beneficial. Aspirin can cause stomach bleeding, but it also thins the blood, which is why some people take it to prevent heart attacks. The line between a side effect and a therapeutic effect depends on why the drug was prescribed.

Side effects range from mild to life-threatening. Common ones include nausea, headache, drowsiness, and rash. Serious ones include liver failure, severe allergic reactions, and cardiac arrhythmias. Some are predictable from the drug's mechanism. Beta blockers slow the heart, so fatigue and cold extremities are expected. Others are idiosyncratic and unpredictable. The frequency and severity of side effects are documented in clinical trials and updated as post-marketing data accumulate. The benefit-risk balance determines whether a drug stays on the market.

Common side effects:

• Gastrointestinal. Nausea, diarrhea, constipation, stomach pain.
• Neurological. Headache, dizziness, drowsiness, insomnia.
• Dermatological. Rash, itching, photosensitivity.
• Cardiovascular. Palpitations, low blood pressure, arrhythmias.
• Metabolic. Weight gain, high blood sugar, electrolyte imbalances.

Patients should report side effects to their prescriber or pharmacist. Some can be managed with dose adjustments or switching drugs.