Drug Interactions: Sorting the Evidence From the Noise
Interactions are common, frequently invisible, and the most dangerous ones involve medicines people buy without advice.
Key takeaways
- Common culprits: NSAIDs, anticoagulants, antidepressants, antibiotics
- Dangerous combination: NSAID plus ACE inhibitor or ARB plus diuretic
- Most useful single change: Tell every prescriber everything you take, including supplements.
- Commonest mistake: Herbal products alongside prescribed medicines without checking.
- See a doctor if any of the signs under "When to speak to a doctor" below apply to you.
What it is
One medicine changing the effect of another, or of food or a supplement. It sits within Medicines Safety. Below are the published figures, in one place.
The numbers, in one place
| Measure | Figure |
|---|---|
| Common culprits | NSAIDs, anticoagulants, antidepressants, antibiotics |
| Dangerous combination | NSAID plus ACE inhibitor or ARB plus diuretic |
| Grapefruit juice | interacts with several statins and other medicines |
| St John's wort | interacts with many medicines including contraception |
What helps
What follows is the practical part: the things guidance actually asks of you.
- Tell every prescriber everything you take, including supplements
- Ask a pharmacist before buying anything over the counter
- Use one pharmacy
What to avoid
A few of these are widely believed. That is precisely why they are here.
- Herbal products alongside prescribed medicines without checking
- Two different NSAIDs at once
- Assuming over-the-counter means interaction-free
Where to go next
- The First Steps With Drug Interactions
- A Q and A on Drug Interactions
- Drug Interactions — the reference entry
- Repeat Prescriptions: Sorting the Evidence From the Noise
- Buying Medicines Online: Sorting the Evidence From the Noise
When to speak to a doctor
Seek urgent help for unusual bruising or bleeding, black tarry stools, vomiting blood, severe dizziness, confusion, or a racing heartbeat after starting a new medicine or supplement. Report suspected reactions through the MHRA Yellow Card scheme.
Where this comes from
How this page was put together
Every figure, recommendation and warning above is taken from the guidance named in Where this comes from. Nothing on this page is inferred, summarised or generalised beyond what those bodies publish: where they give a number, it is their number, and where they are silent, so are we. Pages are rebuilt whenever the underlying record changes. Sign-off by a named clinician, where it has happened, is shown at the top of the page along with the date.
Medical disclaimer
This article is general information, not medical advice. It cannot account for your own history, medication or circumstances. Always speak to a GP, pharmacist or another qualified health professional before making changes that affect your treatment, and never delay seeking medical advice because of something you have read here. In an emergency, call 999.
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