A、Because they are not accustomed to it. B、Because they are not psychologically prepared for it. C、Because their genes differ fr

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. [20] 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. [21] Many people suffer ill-effects from drugs called side-effects even though they take the drugs 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. [22] A patient may also acquire a tolerance for a certain drug, which means the patient has to take ever-larger doses to produce the desired effects. Tolerance may lead to heavy situation, in which a person becomes so dependent on the drug that he or she becomes addicted to it. Addiction causes severe psychological and physical disturbances when a drug is taken away. Finally, drugs often have unwanted side-effects. They usually cause only minor discomfort, such as a skin rash, headache or sleepiness. Certain drugs, however, can produce serious adversary actions.
20.Under what circumstances are drugs approved?
21.Why do many people suffer side-effects from a drug even though they take it as directed?
22.What will happen when patients acquire a tolerance for a certain drug?