Warning: the 9 everyday foods not to eat with medication

We all know there are some medications you can’t drink alcohol with or can only take with food. But many of us don’t realise that many everyday foods can also affect how well our prescription drugs work.  

Everything from bananas and grapefruit to smelly cheeses, salami and leafy greens can potentially have an impact on how medications work.  

The interactions can be particularly hard to avoid if you are taking multiple meds – as do almost half of people aged 65 or over. The numbers taking more than five types of medication a day has quadrupled over the past 20 years, from 12% to 49%, according to one study). Latest figures from the NHS Specialist Pharmacy Service published in January this year revealed 8.4 million patients in England take five or more medicines daily. 

“The high-risk patients for drug/food interactions are those on multiple drugs who have chronic conditions,” says Professor Amira Guirguis, a pharmacist and academic at Swansea University Medical School and spokesperson for the Royal Pharmaceutical Society.  

9 foods you shouldn't mix with medication

“Some foods may reduce absorption of your medication so you don’t get the benefits,” says Prof Guirguis. “Other foods may affect how a drug is metabolised and broken down, so levels of the drug in your bloodstream either go too low or too high. 

“You can avoid some of these interactions if you check the patient information leaflet in your pill or medicine packet which will list any known food/medicine issues and advise you whether they should be taken with food or on an empty stomach.”  

Here are some of the foods that may interact with commonly prescribed drugs. 

1. Bananas  and salt substitutes

Bananas are healthy and a rich source of fibre, vitamin B6, and the minerals potassium and magnesium.  

Although potassium can help lower blood pressure, foods high in potassium can also be problematic if you’re taking blood-pressure-lowering medications such as ACE inhibitors (such as ramipril) and potassium-sparing diuretics (spironolactone, for example).  

Salt substitutes such as Lo-Salt contain potassium chloride and can also increase potassium levels in the body.

“Potassium levels in the blood can occur to varying degrees (mild, moderate, or severe hyperkalaemia), and this can be dangerous, particularly for people taking certain medications like ACE inhibitors and potassium-sparing diuretics,” says Prof Guirgus.  

“That’s why GPs usually monitor potassium levels in blood tests regularly in at-risk patients and adjust/optimise treatment as needed. In clinical guidance, the advice is for these vulnerable patients to avoid potassium-rich foods such as bananas and salt substitutes such as Lo-Salt (which is high in potassium) regardless of quantity.  

“While the occasional banana might not cause a problem for everyone, the safest recommendation for those on these medications is to avoid them to reduce risk,” says Prof Guirgus.  

2. Grapefruit and grapefruit juice

Grapefruit is a common culprit in drug interactions because it interferes with a particular chemical in the liver that metabolises many drugs. 

“Grapefruit juice inhibits the liver enzyme CYP3A4 and reduces its activity,” explains Prof Guirguis. “This enzyme is responsible for the metabolism and breakdown of many medicines. These include the anti-arrhythmic drug amiodarone, the calcium channel blocker nifedipine, the blood-pressure-lowering medicine losartan (an angiotensin receptor blocker) and lipid-lowering drugs (statins) such as atorvastatin.”  

Drinking grapefruit juice with these medicines may either increase or decrease exposure to the drugs’ ingredients which may lead to either undesired/toxic effects or loss of therapeutic effects.  

“This interaction with atorvastatin, for example, can lead to muscle toxicity [a condition called rhabdomyolysis which causes muscle pain and dark urine]. This can be confirmed with a blood test,” she says.

“Taking grapefruit juice with sildenafil, a drug used for erectile dysfunction, can cause severe low blood pressure and flushing. Grapefruit juice with these medicines should be avoided.” 

3. Orange and cranberry juice

Conversely, taking orange juice with some medicines such as iron supplements may improve absorption, says Prof Guirguis.  

On the flip side, acidic juices in general can reduce the effectiveness of the antibiotic penicillin.  

Cranberry juice and grapefruit juice also have the potential to interact with warfarin, magnifying its clinical effects – increasing the risk of bleeding. Ideally both should be avoided, but especially in large quantities.  

4. Milk and dairy products

Milk and dairy products should not be taken at the same time as fluoroquinolone antibiotics – broad spectrum antibiotics used to treat severe infections (eg ciprofloxacin and levofloxacin) and tetracyclines (eg doxycycline).  

“This is because these antibiotics have chemical groups with a positive charge which can easily bind to calcium in dairy products,” Prof Guirguis says. “They can form complexes that make them insoluble, and they won’t be absorbed in the body so won’t work effectively against your infection.  

“If people who take these antibiotics want to eat dairy products, we advise them to take the medicine one hour before they eat dairy products or two hours afterwards. The best advice is not to eat or drink dairy at the same time.”  

5. Leafy greens and chick peas

Leafy green vegetables such as kale, spinach and broccoli contain high levels of vitamin K, needed for blood clotting, as do other foods such as chickpeas, mature cheese, liver, egg yolk, and blue cheese.     

High levels of vitamin K can cause problems for people on the blood-thinning drug warfarin. This is because warfarin works by blocking vitamin K – thinning the blood and reducing the risk of clots. High levels of vitamin K in the diet may counteract this effect.  

Specialist dietitian Clare Thornton-Wood, at the British Dietetic Association, says that too many vitamin K-rich foods reduce the effectiveness of warfarin and therefore increase the risks of clots.  

“You don’t have to stop eating leafy greens – just be consistent in the amount you take,” says Thornton-Wood. “If you normally eat a lot of them that’s fine as your medication dose can be adjusted to fit with this.

"It’s sudden changes in diet that are to be avoided, so don’t binge on them if you don’t normally eat a lot of them.” 

6. Cured, pickled foods, mature cheese and salami 

These all contain tyramine, which is a naturally occurring substance found in fermented, cured, pickled and aged foods such as mature cheeses, salami, smoked salmon, pickled herring, yeast extract, soya bean extract and some beers, lagers, or wines.  

“Monoamine oxidase inhibitor (MAOI) antidepressants, such as phenelzine, interact with tyramine-rich foods, preventing the breakdown of tyramine which may lead to hypertensive crisis [a sudden severe increase in blood pressure],” says Prof Guirguis. “This may in turn lead to cerebral haemorrhage [a type of stroke or bleeding in the brain] so these foods should be avoided.  

“Patients are advised to eat fresh foods and avoid foods such as leftovers or food that looks stale.”  

7. High protein diets (eg. Atkins and Keto) 

The bronchodilator medicine theophylline is used to treat asthma and Chronic Obstructive Pulmonary Disease (COPD) by relaxing the smooth muscles around the airways in the lungs, opening them up and making it easier to breathe.  

“Its therapeutic effectiveness may be affected by the amounts of protein and carbohydrates in food,” says Prof Guirguis.  

“High protein diets may reduce the therapeutic effects of theophylline, reducing its effectiveness, whereas high carbohydrate diets may increase its effects.” 

8. High fibre diets

Some medicines can be negatively affected by eating a high-fibre diet (such as high-fibre breakfast cereals). These include tricyclic anti-depressants such as amitriptyline. “These drugs may fail to relieve depression in patients eating high-fibre diets,” says Prof Guirguis.  

9. Caffeine

Levothyroxine is prescribed for people with an underactive thyroid gland (hypothyroidism) where the thyroid gland doesn’t produce enough hormones.  

Caffeine in coffee and tea can affect absorption, so levothyroxine needs to be taken ideally at least 30 minutes before drinking a caffeinated drink in the morning, or eating breakfast, says the British Thyroid Foundation.  

High caffeine intake (on average more than five cups of coffee a day), together with theophylline, may cause insomnia and heart arrhythmias.