Understanding Drug Interactions

What is a drug interaction?

A drug interaction is when one substance changes the way another works in the body. That substance might be another medicine, a supplement, a herbal remedy, a food or drink, or even alcohol. Interactions can make a medicine less effective, increase the chance of side effects, or occasionally cause new problems. Understanding the basics helps you use your medicines more safely.

The main types of interaction

Interactions broadly fall into a few groups:

• Medicine with medicine — two medicines affecting each other, for example by adding to the same side effect or by changing how one is broken down
• Medicine with food or drink — certain foods, drinks or alcohol can alter how a medicine behaves
• Medicine with supplements or herbal products — vitamins, minerals and herbal remedies can interact, even though they may feel harmless
• Medicine with a health condition — some medicines are less suitable if you have particular conditions

Common examples to be aware of

Without going into specific products, some familiar patterns include:

• Medicines that each cause drowsiness adding together, increasing sedation; this is relevant for some nerve-pain and seizure treatments discussed in our nerve pain explained guide
• Alcohol increasing the sedative or stomach-irritating effects of some medicines
• Certain painkillers affecting other treatments, which is worth checking before combining anything from our pain range
• Some supplements affecting how well a medicine is absorbed

These are illustrations, not a complete list. The patient information leaflet and a pharmacist are the right sources for your specific medicines.

Why the leaflet matters

Every patient information leaflet has a section on interactions and on what to tell your doctor before taking the medicine. Reading this before you start, and again if you add anything new, is one of the simplest ways to avoid problems. Our guide on how to read a patient information leaflet shows where to look.

Keeping a medicines list

If you take more than one medicine, a simple up-to-date list is invaluable. Include:

• All prescription medicines
• Anything you buy over the counter
• Vitamins, supplements and herbal products

Show this list to any clinician or pharmacist you see. They can only check for interactions if they know everything you are taking, including things you might not think of as medicines.

Special situations

Some circumstances increase the chance of interactions and deserve extra care:

• Taking several medicines at once, which becomes more common with age
• Starting a new medicine while already on others
• Using over-the-counter remedies alongside prescription medicines
• Pregnancy and breastfeeding
• Having kidney or liver conditions that affect how medicines are processed

In these situations, a quick check with a pharmacist before adding anything new is well worth the time.

How to protect yourself

A few habits go a long way:

• Always tell your doctor and pharmacist about everything you take, including supplements
• Read the interactions section of each leaflet
• Ask before combining a new over-the-counter product with your regular medicines
• Be cautious with alcohol when on medication
• Use one pharmacy where possible, so your records are in one place

When to seek advice

Contact a pharmacist or doctor if you are unsure whether two things can be taken together, if you notice new symptoms after starting something, or if a medicine seems to be working differently. For anything that feels serious, such as signs of a strong reaction, seek urgent help.

A small amount of checking prevents most interaction problems. You can browse our shop all range or visit the FAQ for ordering questions.

General information only — not medical advice. Always read the patient information leaflet and consult a doctor or pharmacist before starting any medication.