Is it dangerous to take different medicines at the same time?

Reviewed by Dr Christine Clark, pharmacist

What is drug interaction?

Drug interactions can occur when people take two or more different medicines at the same time.

The medicines might be taken for the same illness, such as quinsy (an abscess in the throat), for which a person needs antibiotics and painkillers. Or it may be because they suffer from two different diseases at the same time, such as gastric ulcers and arthritis, for which they need acid neutralisers and medicines for arthritis.

Medicines can sometimes influence each other inside the body, producing an increased effect, extra side effects, or decreased effectiveness of one or more drugs. This is called a drug interaction.

How can two different drugs influence each other's effect?

Nearly all medicines are broken down by specific enzymes in the liver, and then excreted as smaller molecules through the kidneys. Some medicines can affect the way other medicines are broken down in the liver, or are filtered into the urine by the kidneys.

If medicine A inhibits the enzyme responsible for breaking down medicine B, the result can be an excessive amount of medicine B in the bloodstream. This can increase the chance of serious side effects. If a doctor thinks that both medicines are necessary at the same time, they will ensure that the dose of medicine B is reduced, to avoid any adverse effects.

If, on the other hand, medicine A increases the activity of the enzyme responsible for breaking down medicine B, the breakdown will occur much faster and the effect of medicine B will be decreased or lost. In that case, a doctor will increase the dose of medicine B.

Medicines can also influence each other in other ways; for instance, they can affect absorption from the intestines or excretion by the kidneys. In both instances, this can result in effects similar to those described above - too much or too little medicine in the bloodstream, which may lead to serious side effects or little or no effect at all.

If medicines that can have similar side effects are taken together, there may also be a chance that the side effects will be additive. For example, if a sedating anthistamine is taken by someone who is also taking a strong painkiller that can cause drowsiness, for example morphine, then any drowsiness is likely to be increased.

Do I have to tell my doctor about all the medicines I am using?

As long as a doctor or pharmacist has taken into account the effect of taking two or more medicines at the same time it should be perfectly safe. For this reason you should always tell your doctor about all the medicines you are using, including herbal remedies and those bought without a prescription, since drug interactions can occur with these too.

When buying over-the-counter medicines from a chemist you should also remember to check with the pharmacist whether the new medicine is safe to take with any other medicines you are currently using.

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