Medicines act in a variety of ways. Some can cure an illness by killing or halting the spread of invading germs, such as bacteria and viruses. Others are used to treat cancer by killing cells as they divide or preventing them from multiplying. Some drugs replace missing substances or correct low levels of natural body chemicals such as some hormones or vitamins. Medicines can even affect parts of the nervous system that control a body process.
Nearly everyone has taken an antibiotic. This type of medicine fights bacterial infections. Your doctor may prescribe an antibiotic for things like strep throat or an ear infection. Antibiotics work either by killing bacteria or halting their multiplication so that the body's immune system can fight off the infection.
Sometimes a part of the body can't make enough of a chemical. That can also make you sick. Someone with insulin-dependent diabetes, for instance, has a pancreas that can't produce enough insulin (a hormone that regulates glucose in the body). Some people have a low production of thyroid hormone, which helps control how the body uses energy. In each case, doctors can prescribe medicines to replace the missing hormone.
Some medicines treat symptoms but can't cure the illness that causes the symptoms. (A symptom is anything you feel while you're sick, such as a cough or nausea.) So taking a lozenge may soothe a sore throat, but it won't kill that nasty strep bacteria.
Some medicines relieve pain. If you pull a muscle, your doctor might tell you to take ibuprofen or acetaminophen. These pain relievers, or analgesics, don't get rid of the source of the pain — your muscle will still be pulled. What they do is block the pathways that transmit pain signals from the injured or irritated body part to the brain (in other words, they affect the way the brain reads the pain signal) so that you don't hurt as much while your body recovers.