Foods and drugs that should never be mixed

BANANAS, a few leafy greens, half a grapefruit and a class of milk sound like the makeup of a healthy diet, right?

Well, they are, but for those taking certain medications — these foods, as well as your favourite glass of wine or cup of coffee, may actually prevent certain prescription drugs from working, or even cause serious side effects.

Known as food-drug interactions, some foods can make a side effect from medicines worse, or even create a new side effect altogether if not consumed with caution.

“Certain foods have certain chemicals that affect the metabolic processes in the body,” Dr Ross Walker told

“Just because they are natural doesn’t mean they don’t have an affect on the body. There are a whole list of chemicals in food that have an impact on metabolism.

“The big point is that patients need to have a frank discussion with their doctor about everything they are putting in their mouth, whether it be herbs, food, beverages or supplements because they can all interact with medications.”

So what combinations should be avoided?


If you’re enjoying a grapefruit in the morning, but also taking medication to lower your cholesterol, maybe consider taking it off your daily menu. This particular fruit can play around with how your body metabolises certain drugs, meaning more of the drug can end up in your blood stream.

”Grapefruit is a very important food to understand when taking certain medication,” Dr Walker said.

“Statins have a strong interaction with grapefruit, as the fruit blocks a very important enzyme in the liver, which is a drug metaboliser.

“If you take the statin, the levels rise so much in the blood you may get side effects like muscle pain and aches.

“People should avoid grapefruit if you’re on antibiotics as well.”


If you’re taking antibiotics and certain osteoporosis medication, avoid washing it down with a glass of milk. Calcium can interfere with the effects of some antibiotics, so other products like cheese and yoghurt should also be avoided when on medication like tetracycline, ciprofloxacin or alendronate.

“There’s a few antibiotics which milk can block the absorption,” Dr Walker said.

“This is because the calcium in milk binds to the drug in the gut and reduces absorption.”


With bananas being so high in potassium, they can have an impact when taking blood pressure medication. Too much potassium, which can also be found in oranges and leafy greens, can cause irregular heartbeats and palpitations.

“The idea of not eating bananas at all while taking medications that lowers blood pressure and treating heart failure is not overly accurate,” Dr Walker said.

“If people are eating foods with high levels potassium, they may raise their levels to a high point, but its very uncommon.

“Check with your doctor, especially if you do have a kidney problem. But people who think they can’t go near a banana is complete nonsense.”

Black licorice

Taking heart medication and enjoy a piece of licorice as an afternoon treat? The sweet contains glycyrrhizin, which lowers the potassium in your body and can be dangerous to those who with certain heart conditions. Mixing digoxin (treatment for certain heart conditions) with glycyrrhizin can cause irregular heartbeats and may even lead to a heart attack.

“Licorice has a glycyrrhizin acid, which is good for stomach ulcers but also lowers potassium in the body,” Dr Walker said.

“Low levels of potassium can cause cardiac arrest, so people should avoid licorice if on heart medication.”

Herbal licorice extract can also interfere with a host of other medications including insulin, certain antidepressants, oral contraceptives, blood thinners, and some other medications.

Kale and leafy greens

It’s been praised as a super food, but if you’re on blood thinning medication, or drugs that aid the treatment of irregular heartbeats, kale along with other leafy vegetables can interfere in a negative way.

A drug like Warfarin has the greatest impact, which is an anticoagulant normally used in the prevention of the formation of blood clots in the blood vessels.

“Kale is one of many green veggies that has a lot of Vitamin K1, and the popular drug Warfarin works by blocking Vitamin K1,” Dr Walker said.

“So if you are consuming a lot of leafy vegetables that have a heap of the vitamin, it can reduce the effect of Warfarin.

“The newer drugs don’t have any effect at all from kale and other leafy greens, but Warfarin is still the most common, and the one that’s exclusively used in valve replacements.

“So aspirins, for example, leafy greens have no interactions,” he added.


If you suffer from asthma, advice is to avoid coffee because the common side effects associated with caffeine include palpitations, nervousness and excitability.

But according to a study on couples out of the US, it is those hoping to conceive that should really avoid consuming coffee.

The study out of the National Institutes of Health and Ohio State University, showed that in a study of 344 couples who achieved a pregnancy, 28 per cent had a miscarriage in the first eight weeks. But, it also showed that if a couple had more than two cups of coffee in the few weeks before conceiving, there was a 75 per cent increase for miscarriage.

“If you continued the coffee in to the pregnancy, it keeps the risk of a miscarriage quite high,” Dr Ross said.

“But, if you take a multivitamin in the weeks before falling pregnant, it lowers the risk of a miscarriage by 55 per cent, and continuing to take it in the early parts of pregnancy sees a 79 per cent reduction in first trimester.

“Basically, men and women should be making their bodies havens before getting pregnant.”


Lots of medicines often come with a warning to avoid consuming with alcohol. This is because of the pressure alcohol puts on your liver, which is “the major processor in the body.”

“Everything in your mouth goes through the liver,” Dr Walker said.

“There’s a whole heap of drugs that alcohol perpetuates the sedative effect. Alcohol can effect the liver, so it can screw around with how your body works.

“You might get dangerous levels of the drug in your blood stream when paired with alcohol, it’s tiger country when you are dealing with all these things together.”

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