Every medication, even when used in correct dosage and according to correct indication, may apart from the positive effect on some organic system also cause some unwanted adverse effects. Adverse reaction may occur isolated in the oral mucosa or may appear on the oral mucosa within the systemic response to the drug. Given that older people are suffering from multiple diseases and they usually take more medications than younger people, we can more often expect adverse drug reactions among them. Reactions to drugs in the oral cavity are manifested in the form of oral ulceration, pigmentation, lichenoid reaction, erythema multiforme, SJS, TEN, drug-related lupus, drug related pemphigus, stomatitis venenata, gingival hyperplasia, oral candidiasis, burning mouth symptoms, xerostomia, ptialism, swelling and pain of the salivary gland, dysgeusia, and glossitis. There are over 400 types of medicines that can cause adverse reactions in the oral mucosa. Most common among them are sulphonamides, antibiotics, antihypertensives, antidepressants, antiepileptics, NSAR, and immunosuppressive agents. Lesions of the oral mucosa may be the first sign of an adverse reaction to the drug and the oral cavity is the first place where they appear. Therefore, it is very important that doctor of dental medicine recognizes not only these reactions but also their cause, in this case the drug which caused the adverse reaction. After the identification of the drug, the dentist should inform the general doctor and the drug agency about the harmful causes of medication and provide the adequate therapy of the oral lesions. Therapy is most often based on the elimination of a drug or its replacement with another drug. Also, the therapy consists of the local therapy of given adverse reactions.