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 approved only after tests have demonstrated that they are relatively safe when used as directed and when their benefits outweigh their risks. Thus, some very dangerous drugs are approved because they are necessary to treat serious illnesses. Many people suffer ill effect from drug, called side effect, 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 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. A patient may also acquire a tolerance for a certain drug, which means the patient has to take a ever larger doses to produce the desired effect. Tolerance may lead to a situation in which the person becomes so dependent on the drug that he or she becomes addicted to it. Addiction causes severe psychological and physical disturbances when the drug is taken away. Finally, drugs often have on one side effect. These usually cause minor discomfort such as skin rush, headache or sleepiness. Certain drugs, however, can produce serious adverse reactions.

　　Questions 18 to 20 are based on the passage you have just heard.

　　18. Under what circumstances are drugs approved?
　　 19. Why do many people suffer side effects from a drug even though they take it as directed?

　　20. What would happen when patients are acquired of tolerance for a certain drug?