Purified active plant constituents were isolated and assessed for their pharmacological activities that constitute a basis of modern drug development. The situation with herbal supplements is different because the extract or dried herb or mixture of herbs contains several substances beside the beneficial one(s) that might produce drug interaction with the conventional medicine(s). Most patients are misinformed and believe that anything "natural" must be safe. This article is focusing on plant-based substances referred as dietary supplements (DS). Examples of reported drug interactions and contraindications associated with DS with two case studies are presented. As supplements are typically not prescribed, many doctors seem to have no interest in drug-DS interactions since a typical medical history of the patients does not include any questions about self-prescribed remedies of this nature. Rather, patients are left alone when they are tempted to try this or that DS and tend to rely on advice from friends, or on material they read on internet. A better quality control, compliance, public awareness and healthcare professionals vigilance for potential interactions are needed. It is of utmost importance to appreciate the impact of supplements on different stages of pharmacokinetics, especially on drug absorption and metabolism.