Pharmacology

A drug is any substance that alters how the body works. Useful drugs — medicines — treat illness, relieve symptoms or prevent disease. Painkillers dull pain, and antibiotics cure bacterial infections. Drugs may be taken by mouth, by injection or in other ways, and every drug has a correct dose: too little does nothing, too much can harm.

Antibiotics (such as penicillin) kill bacteria or stop them multiplying, without harming your own cells — which is why they revolutionised medicine. Crucially, they work only against bacteria, not viruses, so they do nothing for a cold or flu.

Overusing antibiotics has led to antibiotic resistance: bacteria that survive the drug pass on their resistance, producing strains the antibiotic can no longer kill. This is why doctors avoid over-prescribing them and patients should finish the full course.

Some drugs are misused for their effects rather than to treat illness, and many cause harm. Depressants like alcohol slow the nervous system, slowing reactions and damaging the liver over time. Stimulants like nicotine (in tobacco) speed it up; tobacco smoke also damages the lungs and causes cancer. Many misused drugs are addictive — the user develops a dependence and suffers withdrawal without them.

Before a new medicine can be used, it goes through years of testing to make sure it is safe and works. It is first tested on cells and animals, then in clinical trials on people — starting with small numbers to check safety and the right dose, then larger groups to check it is effective. Trials often use a placebo (a dummy with no drug) for comparison, so any real effect of the medicine can be told apart from the patient simply expecting to feel better.

Not all medicines treat illness — vaccines prevent it. A vaccine contains a weakened or dead form of a pathogen (or part of it), which triggers the immune system to make antibodies and, crucially, memory cells — without causing the disease. If the real pathogen later invades, the memory cells respond so fast that the person does not fall ill: they are immune. When enough of a population is vaccinated, the disease can no longer spread easily — herd immunity — protecting even those who cannot be vaccinated.