Sometime when you take a common drug, you may have a side effect. That is, the drug may cause some effect other than its intended one. When these side effects occur, they are called adverse reactions. Whenever you have an adverse reaction, you should stop taking the drug right away. Ask your pharmacist whether he can suggest a drug that will relieve the symptoms but that will not cause the adverse reaction. If an adverse reaction to a drug is serious, consult your doctor for advice at once. Drugs that are safe in the dosage stated on the label may be very dangerous in large doses. For example, aspirin is seldom thought of as dangerous, but there are many reports of accidental poisoning of young children who swallow too many for their young bodies to handle. In adults, excessive use of some pain-killing drugs may cause severe kidney （肾脏）damage. Some drugs for relief of stomach upsets, when taken in excess, can cause an upset in the body's secretion of enzymes, perhaps causing serious digestive problems. You should never use any over-the-counter drug on a regular, continued basis, or in large quantities, except on your doctor's advice. You could be suffering from a serious illness that needs a doctor's care. Each drug you take not only acts on the body but may also alter the effect of any other drug you are taking. Sometimes this can cause dangerous or even fatal reactions. For example, aspirin increases the blood-thinning effect of drugs given to patients with heart disease. Therefore, a patient who has been taking such a drug may risk hemorrhage if he uses aspirin whenever he gets a headache. Before using several drugs together you should ask your doctor and follow his advice. Your pharmacist can tell you whether certain drugs can safely be taken together. Alcohol may increase the effect of a drug. Sleeping pills and antihistamines are two types of drugs that combine with alcohol to produce drowsiness. When taking any drug, you should ask your doctor whether drinking alcohol could be dangerous in combination with medicine. Experts believe there is a relationship between adult abuse of legitimate medicines and the drug culture that has swept our country. You can do your share to reduce the chances that your children will become part of the drug culture by treating all medicines with respect. Always let your children know that medicines and drugs should not be used carelessly. What are side effects according to the passage?