Abstract
Pharmacovigilance tell us the side effects of the drugs after introduce to the market and used by patients, hence some drugs e.g., thalidomide when used to treat the emesis in pregnant women appear the phocomelia which characterized by abnormal development of limbs where the long bones (like: humerus - femur) or either shortened or absent causing the hands and feet to be attached close to the body, this is due to isomer of thalidomide (S) which is teratogenic but the (R) is a sedative and more safe due to the pharmacovigilance of thalidomide so that the pharmacovigilance led us to use (R) isomer but not (S) isomer in medicine. Captopril appears to cause cough in patients when used as an antihypertensive drug, due to the SH group of captopril. Hence, captopril was modified to enalapril to avoid the cough symptom that appears when using captopril. Cimetidine is H2 antagonist, the first H2 antagonist modified from histamine to used treatment peptic ulcer, the pharmacovigilance tell us with side effects of cimetidine, the major of them decrease the blood cell counts especially WBCs (agranulocytosis), so the cimetidine modified to synthesis of famotidine which is less side effect than cimetidine.