A side effect is an undesirable secondary effect of a drug or other medical treatment, distinct from its intended therapeutic action. Every active medicine carries the potential for them, from the mild, dry mouth, drowsiness, nausea, to the serious, organ damage or allergic reactions. In nutrition, side effects matter with supplements and fortified products: iron causes constipation, high-dose vitamin C causes diarrhoea, and herbal products interact with prescriptions. The likelihood and severity are weighed against benefit in every prescribing decision, and patients are counselled on what to expect and what warrants a call to the doctor. Reporting systems collect side-effect data after marketing, since rare harms only emerge when millions use a product.