Drugs are chemical substances obtained naturally, synthetically, or through genetic engineering. When administered in a living body, drugs produce numerous biological effects that may be classified as desirable or therapeutic or undesirable. The main objective of a desirable effect is to treat the disease. All other effects experienced alongside the desirable effect are considered undesirable, which can be further classified into side effects and toxic effects. Side effects are generally nondeleterious. They are the drugs' unwanted and unavoidable pharmacodynamic effects. Drugs like atropine used to reduce mucous secretions can cause side effects like dry mouth due to the same action that produces the therapeutic effect. Toxic effects are deleterious and categorized as pharmacological, pathological, or genotoxic. Pharmacological effects of drugs arise from excessive or prolonged drug usage and generally disappear as the drug is biotransformed or excreted, reducing its concentration in the body. Pathological and genotoxic effects are, in many instances, repairable, but if the effects are very severe, they may lead to cancer or death.