Drug Interactions: Separating Myth From Guidance

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 the guidance actually says

Here is what guidance actually publishes. The rest of the page is measured against it.

Where people go wrong

Common beliefs that the figures do not bear out.

• Herbal products alongside prescribed medicines without checking
• Two different NSAIDs at once
• Assuming over-the-counter means interaction-free

What to do instead

• Tell every prescriber everything you take, including supplements
• Ask a pharmacist before buying anything over the counter
• Use one pharmacy

The part that is genuinely uncertain

Guidance changes, and it changes because evidence does. The figures above reflect current UK guidance from the bodies named at the foot of this page; where those bodies revise their position, theirs is the account to follow rather than this one.

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

• NICE — British National Formulary interactions
• MHRA — Yellow Card scheme

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.