HOW DRUGS WORK
To understand how these interactions occur, it is essential to know what happens to a drug after it is swallowed, the most common way of taking medication. Generally, the drug reaches the stomach and is absorbed through the stomach wall into the bloodstream. Once in the blood, most molecules of the drug are carried along, loosely connected to proteins, until they reach their target tissues, Many drugs work by linking up to specific proteins called receptors on the surface of cells.

When the drug has performed its function, it has to be removed from the body. Most drugs are broken down in the liver and then eliminated from the bloodstream by the kidneys, which filter the remnants of the drug into the urine.

Interactions, can occur in a number of ways, with the end result that either too high a level of medication is found in the blood, leading to increased side effects, or too little of the drug is present and it will not work effectively.

TYPES OF INTERACTION

Two drugs taken at the same time may affect the same receptor site on the cell surface. The medication that has the strongest attraction to the receptor will prevent the other drug from having an effect. Alternatively, one drug may interfere with the effectiveness of another, either delaying or reducing it. Delayed absorption makes a drug less effective.

Many drugs can increase the levels of the enzymes in the liver responsible for breaking down other drugs taken at the same time. This leads to lower concentrations of the affected medication in the blood, reducing its potency. This action is known as enzyme induction. When the drug causing the induction is withdrawn, levels of the second drug can increase dramatically, causing harmful effects.

SERIOUS INTERACTIONS

Many drug interactions are harmless, and even those that are potentially more serious will only occur in a small minority of patients. However, there is a number of more serious interactions, involving commonly prescribed drugs, which are important. Some of the most significant follow.

The effectiveness of the oral contraceptive pill can be reduced by antibiotics and some drugs for epilepsy. Warfarin, a drug commonly used for thinning the blood, can be affected by some antibiotics, analgesics, drugs used to treat cholesterol, and epilepsy drugs. Antidepressants and tranquilizers are among the drugs acting on the central nervous system. They can have their sedative effects increased by some of the stronger analgesics available.

These are just a few of the hundreds of possible interactions that can take place. It would be very difficult for individual doctors to know all potential interactions, but almost all doctors use computers to generate prescriptions, and many of these interactions are highlighted by the computer systems.

Interactions can also take place between prescription medicines and those bought over-the-counter from a pharmacist. This also applies to herbal medicines and homeopathic preparations. It is important to inform your doctor if you are taking any such medicines because it may influence her prescribing decision.

HOW A DRUG WORKS

A drug taken orally reaches the stomach and is absorbed through the stomach walls into the bloodstream and carried to target tissues. It then has to be removed from the body, and is broken down in the liver and eliminated by the kidneys.

There are a number of ways of introducing drugs into the body. Most drugs are taken orally in the form of pills, capsules, or liquids. Intravenous injection enables a drug to take effect very quickly because it enters the bloodstream and is circulated to the part of the body where it is needed. Drugs act in a variety of ways. Some simply kill off invading organisms such as bacteria, viruses, and fungi. Others alter the effects of body chemicals. Some may affect a part of the nervous system that controls a particular process.