In order for a chemical to be considered a drug, it must have the capacity to affect how the body works. No substance that has the power to do this is completely safe. Drugs are only approved after tests have demonstrated that (1)____________________. Thus some very dangerous drugs are approved because they are necessary to treat serious illnesses. Many people suffer ill effects from drugs called side effects, even though they take the drug exactly as directed. The human population contains a great variety of genetic variation, but drugs are tested on just a few thousand people. When a particular drug is taken by millions, some people may not respond in a predictable way, even though the drug has been tested. (2)_____________________, which means the patient has to take ever larger doses to produce the desired effect. Tolerance may lead to habituation, in which the person becomes so dependent on the drug that he or she becomes addicted to it. Addition causes severe psychological and physical disturbances when the drug is taken away. Finally, drugs often have unwanted side effects. This usually causes only minor discomfort, (3)___________________. Certain drugs, however, can produce serious adverse reactions.
