Occasionally the food we eat can interfere with the way drugs are supposed to work. Now scientists are trying to harness these effects to boost the efficacy of treatments.

It was an embarrassing problem. And, after five hours with a constant erection, a painful one.

Doctors were initially baffled by the predicament of a 46-year-old man who turned up at a hospital emergency room in Tamilnadu, India. He had taken the drug sildenafil – better known as Viagra – for erectile dysfunction before having sex with his wife. Yet, even though he had taken the drug well within prescribed doses, nothing he could do would deflate the situation.

When questioned further, his doctors learned the man had also drunk a hefty slug of pomegranate juice beforehand. They treated him with a jab to counteract the effects and advised him to lay off the juice in future – it had, they concluded, inadvertently boosted the potency of the drug he was taking.

The case is just one example of how the food we eat can interact with medications in unpredictable ways. There is a rich body of medical literature detailing bizarre – and sometimes worrying – occasions when foods have combined with drugs to produce unusual side-effects. While most of this exists as anecdotal reports of individual or small clusters of cases, there there is also now a growing body of research detailing the several ways that foods, drinks and herbs can interact with pharmaceuticals inside the human body. Grapefruit, for example, has long been known to boost the power of a wide range of drugs, increase the risk of side effects or even make normal doses toxic. Fibre-heavy foods, on the other hand, can make some drugs work less effectively.

Although pharmaceuticals usually go through decades of development and testing to ensure they are safe and effective, there are thousands of types of drugs on the market and millions of combinations of foods they could be paired with. Scientific reviews suggest food interactions can be a major threat to safe and effective oral pharmacotherapy. Experts are only now starting to track these interactions systematically, and some even hope to harness these combinations to make drugs work better than they would on their own.

"Most drugs are unaffected by food," says Patrick Chan, professor of pharmacy practice and administration at Western University of Health Sciences in California. "In the certain cases where certain drugs are affected by food, those are the ones we need to watch out for."

Both the US Food and Drug Administration and the European Medicines Agency require that drugs undergo food-effect testing, with trials involving people who have either fasted or eaten a high-calorie, high-fat meal – two slices of toast with butter, two slices of fried bacon, two fried eggs, some hash-brown potatoes, and a large glass of whole milk.

But it is almost impossible for them to test everything. And human metabolism is complicated, says Jelena Milešević, a research associate at the Center of Research Excellence in Nutrition and Metabolism in Belgrade, Serbia. "It's like a little factory and you have a lot of inputs and a lot of outputs," she says.

Once all the chemical reactions of the body, of the food, and of the drug are meshed together, "it's huge, and it's very difficult to separate", says Milešević, who has been researching how vitamin D affects drugs in the body, and vice versa.

Food can affect the medication we take in two ways: it can interact with the active ingredients in the drug, or it can alter how our body itself reacts to a drug.

Some food-drug combinations have been known since the 1980s. One well-known example is how grapefruit and grapefruit juice can interfere with drugs, including some cholesterol-lowering drugs such as statins, along with medicines used to treat high blood pressure, such as nifedipine and felodipine. It also interacts with cyclosporine, a drug used to suppress the immune system to avoid organ rejection after an organ transplant, and several other pharmaceuticals widely used by patients around the world.

Grapefruit can also increase how much of a drug is available to be picked up in the bloodstream, and so make the potency of a dose higher, including some anti-malarial drugs like artemether and praziquantel, and antiviral drugs like saquinavir. It does this by inhibiting a key enzyme called cytochrome P450 3A4, which is responsible for breaking down many different types of medicines. This can lead to the drugs accumulating to levels where they can even become toxic, such as with the erectile dysfunction drug sildenafil, which is sold as Viagra.

"Without this enzyme, the drugs stay longer in the body and the concentrations can become very toxic," says........

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