Since virtually all medicines are taken by mouth, it’s not surprising that the contents of the stomach can have a significant effect on how drugs are absorbed. A recent review of the problem showed that in general, the presence of food will delay absorption, resulting in “a loss of therapeutic efficacy” (Med Clin North Am, 1993; 77: 815-29). The authors also admit that unexpected toxic reactions can occur.

Grapefruit juice contains a unique set of flavonoids that can affect the 3A4 cytochrome, a liver enzyme involved in the metabolism of certain heart drugs and antihistamines. In one study, grapefruit was found to triple blood levels of the heart drug felodipine in the circulation, risking an ischaemic attack (Lancet, 1991; 337: 268-70).Grapefruit can also affect the metabolism of the antihistamine terfenadine, provoking dangerous, if not deadly, heart rhythm abnormalities (JAMA, 1993; 269: 1535-6). People have died from terfenadine poisoning after taking as little as two glasses of grapefruit juice with the medicine.

Liquorice can deplete potassium from the body and so is dangerous if taken with potassium reducing drugs, such as diuretics. The combination has been known to cause paralysis (N Eng J Med, 1991; 325: 1223-7). There are also anecdotal reports of irregular heart rhythms from the interaction of liquorice with heart drugs such as digoxin, leading even to cardiac arrest.

Salt substitutes (which are rich in potassium) may lead to dangerously high potassium levels for cardiac patients taking ACE inhibitors.

Cardiac drug warfarin reacts with broccoli, cabbage, Brussels sprouts, asparagus, lettuce and avocado. The high levels of vitamin K in these foods can hinder the anticlotting effect of the drug (Ann Int Med, 1994; 121: 676-83).

Vitamins are poorly absorbed in the presence of certain drugs. Drugs such as the tetracycline based antibiotics are known to bind with iron, magnesium and zinc supplements and so prevent the drug’s absorption. Another class of antibiotics, the quinolones, are also malabsorbed in the presence of high levels of calcium, zinc or iron.

Iron supplements can reduce the effectiveness of a wide variety of drugs: antibiotics, the thyroid treatment thyroxine, the heart drug captopril, the blood pressure drug methyldopa, the anti Parkinson drugs levodopa and carbidopa and the vitamin folic acid (Br J Clin Pharmacol, 1991; 31: 251-5). Vitamin E is gradually becoming accepted as a useful natural thinner of the blood, so caution is advised if another anticoagulant, such as warfarin or aspirin, is taken at the same time. On the other hand, another nutritional supplement, coenzyme Q10, has been found to reduce the anticlotting properties of warfarin (Lancet, 1994; 344: 1372-3).

High fibre diets have been shown to reduce the absorption of digoxin (J Clin Pharmacol, 1989; 29: 529-32); they can also interfere with tricyclic antidepressant therapy (J Clin Pharmacol, 1992; 12: 438-40) a particularly relevant finding since constipation is one of the side effects of the tricyclics.

High fat diets can boost the absorption of certain asthma drugs containing slow release theophylline and make their side effects more hazardous. On the other hand, barbecued food can eliminate these drugs more rapidly (Clin Pharmacol Ther, 1978; 23: 445-9).

Too little food is also potentially hazardous. It was only recently discovered that fasting makes the common painkiller paracetamol more toxic, and greatly increases the risk of liver damage (JAMA, 1994; 272: 1845-50).

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Written by What Doctors Don't Tell You

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