Adverse effects to drugs in general and antimicrobials in particular are common and expected problems faced during the care of patients. Most of them are due to intolerance, colateral and side effects, allergy, idiosincracy, overdose or untoward drug interactions. During antimicrobial use for terapeutic or preventive purpose in documented or suspected infections unexpected adverse effects or harmful consequences can ocur. This paper reviews 7 situations in which this may happen: Antimicrobial makes infectious disease or its damage worse (i.e., early antibiotic treatment of Escherichia coli O157:H7 intestinal infection); antimicrobial exerts its effect and by doing so harms the host (Jarisch Herxheimer reaction in the treatment of syphilis); antimicrobial by itself is insufficient to cure the infection (undrained abscesses, prosthesis-related infection without removal of foreign body); antimicrobial treatment for non significant or self limited infections, side-effect risk outbalances benefits (treatment of asymptomatic bacteriuria in the low-risk elderly); early antimicrobial treatment in chronic infection brings no more benefit than delayed therapy and jeopardize future options (HIV disease, hepatitis B); antimicrobial treatment restores impaired immunity triggering inflamatory response harmful to the host (antiretroviral therapy during early phase of tuberculosis treatment); antimicrobial treatment for untreatable infections or non infectious conditions (Chagas´ disease, alergic or autoimmune conditions). Besides, especial populations, more prone to adverse effects to drugs are: the elderly, patients with renal or hepatic diseases, HIV disease, pregnant women and, in general, patients receiving several concomitant medications