Foods and medicines are more than just things to satisfy cravings or make you feel better – they’re complex chemicals that nourish the body and help it thrive. But when these chemicals come into contact with each other (in the gut, for example) they can interact, sometimes leading them to not work as intended or triggering uncomfortable side effects.

If a medication absolutely shouldn’t mix with a particular food or drink, you’ll invariably find this pointed out on the prescription label. But it can be easy to miss or forget about these warnings once you get into a routine of taking your medication, and less severe interactions may not even be flagged. The same can be said for foods that block the absorption of supplements: because they’re not really regulated like medicines, it can be hard to find information.

So, to help you wrap your head around the often-complex science of how food, supplements and medications can interact, we asked Ian Budd, pharmacist at Chemist4U, to talk us through some of the most common ones to look out for. Here’s what he had to say.  

“Many medications interact with food and drink, altering how they are absorbed or metabolised,” says Budd. “One of the most common interactions is dairy products reducing tetracycline and ciprofloxacin antibiotic absorption.”

In the UK, tetracyclines are used to treat conditions such as bacterial infections, rosacea, acne and H. pylori. A common brand used by the NHS is Tetralysal 300. Ciprofloxacins, on the other hand, are used to treat infections such as pneumonia and skin, bone eye and ear infections. Brand names prescribed in the UK include Ciproxin, Ciloxan and Cetraxal.

These drugs can be affected by dairy consumption; in some cases the calcium in the dairy product binds with the antibiotics, preventing them from entering the bloodstream. To avoid this, it’s commonly recommended to wait around three hours after taking your antibiotic before enjoying any dairy. Basically, don’t wash them down with a big glass of milk. 

Warfarin is a common medication used to treat blood clots and reduce the risk of heart attacks and stroke, but foods rich in vitamin K (such as spinach, kale, broccoli and blueberries) can stop it from working effectively.

Vitamin K and warfarin can work against each other: warfarin works to ‘thin’ the blood and stop clots from forming, while vitamin K helps the blood to clot.

That doesn’t mean people with warfarin should stop eating vitamin K altogether, as it’s important for overall health. It just means that people on the drug need to keep their intake of vitamin K stable, so the drug dosage is accurate. 

Those on antidepressants should avoid taking 5-HTP (or 5-hydroxytryptophan), a supplement found in most health stores, without speaking to their doctor first.

This is because 5-HTP can increase serotonin levels in the brain. While that’s good news for people looking to boost their mood and sleep, antidepressants (ie selective serotonin reuptake-inhibitors) already stop serotonin from being reabsorbed after it’s been used, so adding even more serotonin to that equation can lead to too much. 

This can trigger a condition known as serotonin syndrome, a potentially life-threatening condition that can cause symptoms including hot flashes, high blood pressure and confusion.

It’s worth noting that 5-HTP cannot be found in the foods we eat, so you don’t need to worry about consuming too much of it from whole food sources. Our bodies create 5-HTP using tryptophan (found in eggs, milk, salmon and soy), but eating lots of tryptophan doesn’t lead to a big increase in 5-HTP in the way that a supplement can. 

Grapefruit is the big bad wolf when it comes to food and medication interactions. “Grapefruit inhibits the CYP3A4 enzyme in the liver and intestines, which reduces drug breakdown,” Budd says. “This can lead to higher-than-intended drug levels in the bloodstream, increasing the risk of side effects such as muscle damage with statins or excessive sedation with antidepressants.”

It’s not just grapefruits that can be problematic: “While grapefruit is the most well-known culprit, Seville oranges and pomelos can have similar effects,” says Budd. “Even a small amount can be significant, and the interaction can last up to 24 hours.” This is due to the presence of a chemical compound known as furanocoumarins that have been linked to the bitter taste of these fruits.

Antidepressants that can be affected by grapefruit consumption include sertraline, trazodone, fluoxetine, fluvoxamine and paroxetine. Most medications that can be influenced by grapefruit will have it indicated on the label. 

Many of us like to take our supplements in the morning, but if you’re taking an iron supplement soon after your morning cup of tea, now’s the time to stop.

This is because the tannins found in tea – a type of polyphenol – can bond to the iron in certain foods or supplements, inhibiting its absorption. This is particularly true for plant-based (non-heme) iron sources, so you’ll want to be extra careful if you’re following a vegetarian or vegan diet. 

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