What a drug interaction is
A drug interaction happens when one substance changes the effect of a medicine. The medicine may become stronger and cause side effects, or weaker and stop working. Interactions can happen between two medicines, between a medicine and food or drink, or between a medicine and an illness. They are more common today because many people see different doctors and take several medicines for diabetes, blood pressure, heart disease and pain at the same time.
Who is most at risk
Anyone can have an interaction, but some groups are at higher risk. Elderly people often take many medicines and clear them more slowly. People with kidney or liver disease handle medicines differently. Pregnant and breastfeeding women need to consider the baby too. Children may be given adult products or combined cold remedies by mistake. People who use over-the-counter medicines or herbal products without telling their doctor are also at risk.
Painkillers and other medicines
Common anti-inflammatory painkillers (NSAIDs), such as ibuprofen and diclofenac, are behind many interactions. They can raise blood pressure and reduce the effect of BP medicines, harm the kidneys especially together with certain BP medicines and water pills, and increase bleeding risk with blood thinners, aspirin and some antidepressants.
People with kidney disease, heart failure, stomach ulcers, or those taking blood thinners should ask a doctor or pharmacist before using any painkiller, including creams and gels. Paracetamol is often a safer choice for many people, but it too must be used as the label or pharmacist advises.
Examples doctors watch for
Doctors and pharmacists check for many interactions every time they prescribe or dispense a medicine. Some well-known examples are listed below. This is not a full list, and the decision to combine medicines, or to change timing to avoid a problem, always belongs to your doctor.
- Blood thinners such as warfarin with certain antibiotics, antifungals or painkillers, which can raise bleeding risk
- Some cholesterol medicines (statins) with certain antibiotics and antifungals, raising the risk of muscle damage
- Sleeping pills, sedating antihistamines, opioid painkillers and alcohol together, which can dangerously slow breathing
- Antacids, iron and calcium with thyroid tablets and some antibiotics, reducing absorption
- Some antidepressants with tramadol or migraine medicines, which can cause serotonin syndrome
- Diabetes medicines with alcohol, which can cause low blood sugar
Food, drink and herbal products
What you eat and drink can matter too. Grapefruit and its juice can increase levels of certain heart and cholesterol medicines. Large sudden changes in green leafy vegetables can affect warfarin. Milk and dairy reduce absorption of some antibiotics. Alcohol interacts with a long list of medicines, including painkillers, sleeping pills, antidepressants and diabetes medicines.
Herbal and traditional remedies are not automatically safe just because they are natural. Some can affect bleeding, blood sugar, blood pressure or liver function, and a few change how the liver handles other medicines. Always tell your doctor about any such products you use, including powders, kadhas and supplements.
Cold and cough products
Many cold, flu and cough products combine several ingredients, such as paracetamol, an antihistamine and a decongestant. Taking two such products, or one along with a separate paracetamol tablet, can lead to overdose without realising it. Decongestants can raise blood pressure and heart rate. Read the generic names on every label and ask your pharmacist, especially for children, elderly people and people with high blood pressure or heart disease.
How to protect yourself
Most interactions can be avoided with simple habits. The most useful step is to keep one complete, updated list of everything you take, with generic names and strengths, and show it every time you get a new prescription or pick up something from the chemist.
- Include prescription medicines, over-the-counter products, vitamins and herbal remedies
- Use one regular pharmacy where possible, so records are in one place
- Ask 'Will this interact with my other medicines?' before starting anything new
- Do not stop or change prescribed medicines on your own to avoid an interaction
When a change is needed
Sometimes doctors combine medicines that interact because the benefit is greater than the risk, and they plan extra monitoring, such as blood tests for kidney function, potassium, blood sugar or clotting. Keep these test appointments. If you notice new symptoms after starting a medicine, such as unusual bleeding, dizziness or muscle pain, consult a doctor or pharmacist rather than stopping on your own.
See a doctor urgently if
- Unusual bleeding, black stools, or blood in urine or vomit
- Severe drowsiness, slow breathing or difficulty waking someone
- Unexplained muscle pain or weakness with dark urine
- High fever, confusion, shaking and fast heartbeat after starting a new medicine
- Fainting, very low blood sugar, or a very fast or irregular heartbeat
Tests that may help
This article is general health information, not medical advice. Do not start, stop or combine medicines on your own — always follow your doctor's prescription and ask your pharmacist if unsure.