I suspect many of my patients self-medicate by taking herbal medicines or dietary supplements, which is fine, except they don't tell me. And that can cause problems because although these medicines may be ‘natural' - after all they come from health-food shops - they are not necessarily safe, particularly when it comes to drug interactions.

This is where one interferes with the action of another, and is surprisingly common. In fact, it has been estimated to be the cause of around five per cent of hospital admissions. It is not just prescribed drugs that cause problems. Dietary supplements and herbal medicines contain chemicals that can interact adversely with prescription medication.

For example, if you have high cholesterol and are on a statin such as simvastatin, I would expect your GP to warn you against consuming grapefruit or its juice as this can increase the risk of side-effects. If you use St John's wort - a popular herbal remedy for anxiety and mild depression - and don't