Processed and fresh grapefruit juice can affect the action of some statin-based cholesterol-lowering drugs (such as Zocor and Lipitor). Scientists believe that super nutrients in the juice called “furanocourmarins” suppress the action of CYP34A, an enzyme in our digestive system and liver which metabolises and eliminates certain statins from our bodies.

If you drink grapefruit juice, you may have more statins in your bloodstream than you need which may increase the risk of potential side-effects.

CYP34A plays a role in the metabolism of about half of all medications that we take in the UK, so grapefruit juice may affect the action of other drugs such as benzodiazepine (Valium) or nifedipine (in blood pressure and angina pills such as Adalat and Cardilate).

Juices may block drug absorption too

Grapefruit juice - and perhaps other juices including apple and orange - also may block the intestinal absorption of some drugs, such as antihistamines, meaning that only half the dose makes it in to your bloodstream. Researchers at the University of Western Ontario believe that up to 50 drugs may be affected by the juices, but say that more work needs to be done.

What to avoid with antibiotics...

Most fruit juices and all colas can reduce the effectiveness of antibiotics. You should also avoid milk and dairy foods if you're taking taking tetracycline (a broad spectrum antibiotic) because the calcium and magnesium they contain grab on to the drug and form a compound which can not be broken down. You end up losing both the minerals and the drug as they whiz through your intestines unabsorbed.

...and if you're taking warfarin

Avoid high-fat foods as too much fat lowers the amount of drug that you absorb. Kale and dark green vegetables are also off the menu as they contain the blood-clotting vitamin K which counteracts the medication's blood-thinning effects.

Drugs that affect food absorption

While foods can mess up the action of some medications, certain drugs can interfere with the absorption of some nutrients. For example, cimetidine (Tagamet and Zantac) helps to treat stomach ulcers and heartburn by lowering the acidity in our stomachs. This can have the knock-on effect of lessening our ability to absorb the energy-boosting mineral iron since it needs an acidic environment for optimal absorption.

If you have to take these drugs for a long period, you need to be extra vigilant about getting enough iron-rich foods like oily fish, lean red meat, eggs and dark green vegetables.

Amanda Ursell is the Times nutritionist

www.amandaursell.com

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