Every drug, apart from the desired effect might have an unwanted side-effect. Side effects of the drugs may occur if the drug is used in standard doses and forms of application. Drug reactions can occur immediately after the administration of the drug or after a few years of use. Adverse reactions to drugs can manifest in different parts of the body. In the oral cavity, every part of the oral mucosa can be affected which may be a result of medications taken either locally or systemically. Oral drug reactions are characterized by a variety of clinical symptoms and lesions and therefore are not easy to recognize. Adverse effects of systemic medications might be numerous, but frequently noted are xerostomia, taste disturbances, burning mouth symptoms, oral ulcerations, erythema multiforme, gingival hyperplasia, lichenoid reaction, pemphigus, fixed drug eruption, angioedema, contact allergy. Dentists must be aware of pharmacologic actions, therapeutic use and adverse effects of each drug, knowing how to control the consequences of drug therapy. Therapy is based on the immediate exclusion of the offended drug and treatment of oral lesions, most commonly by use of local or systemic corticosteroids.