Meds and you

Updated 05 April 2017

Foods that influence the effect of medications

A variety of foods can have an influence on how effectively and safely certain medications work in the human body. DietDoc has more.


In prior articles relating to interactions that may occur between foods and medicinal drugs or medications, we have discussed some of the drugs that can interfere with nutrient uptake, and considered those medications that are linked to weight gain or weight loss. The opposite can also occur, when certain foods affect how effectively and safely specific medications work.

Pressor agents found in foods

Certain chemicals called "pressor agents" (tyramine, dopamine, histamine and phenylethylamine) are known to enhance the effect of monoamine oxidase (MAO) inhibitors such as tranylcypromine which is used to treat depression. Under normal circumstances these pressor chemicals in foods are broken down by enzymes like MAO or diamine oxidases. If a patient is, however, taking an MAO inhibitor like tranylcypromine which inhibits the action of the MAO enzyme, then the pressor agents can cause a variety of negative effects including a hypertensive crisis. The latter severe reaction is characterised by an increased heart rate, flushing, and headache, and may lead to the patient suffering a stroke, and even death (Mahan et al, 2011; MIMS, 2011)

Medications containing selegiline which is used in the treatment of Parkinson’s Disease, should not be used together with foods that contain the pressor agent tyramine. Foods that contain pressor agents are also contraindicated when patients are being treated with the antibiotic linezolid (Mahan et al, 2011; MIMS, 2011).

Foods that contain pressor agents

Patients taking tranylcypromine, selegiline (in doses higher than 10 mg per day) and linezolid must avoid the following pressor agent-containing foods:

  • Strongly flavoured, aged cheese - cheddar, blue, Gorgonzola or Stilton cheeses- and any cooked dishes made with aged cheese
  • Aged meats - dry sausage like salami, ‘droë wors’ or biltong
  • Soy sauce
  • Fermented soya beans, soya bean paste, teriyaki sauce
  • Tofu or fermented bean curd, tempeh (a fermented soy bean product from Indonesia)
  • Miso (a traditional Japanese seasoning made by fermenting rice, barley and/or soy beans, usually used as a paste)
  • Fava (broad) beans, snow pea pods (contain dopamine)
  • Sauerkraut (pickled, fermented cabbage)
  • Concentrated yeast extracts (Marmite)
  • Banana peels
  • Meat, fish or poultry stored for longer than 3-4 days in the fridge can produce pressor amines
  • Red or white wine (use less than 60 ml a day)
  • Coffee, cola nut extract and any foods/drinks that contain caffeine which is a weak pressure agent. Quantities exceeding 500 mg caffeine a day can produce pressor agent reactions.  Avoid caffeinated energy drinks.
  • Beer, including non-alcoholic beer (do not exceed 2 x 300 ml bottles a day)
  • Liqueurs or distilled spirits such as brandy (not more than 2 tots a day)

(Please note that the prohibition on the intake of alcoholic beverages in this section relates to their content of pressor agents and not to their alcohol content which can also affect how medications work in the human body).

(Mahan et al, 2011; MIMS, 2011)


Caffeine is also a stimulant and when taken in larger doses can influence the effect of amphetamines, methylphenidate (used in medications for ADHD) or theophylline (used in cough syrups for example), resulting in nervousness, shakiness and insomnia (Mahan et al, 2011; MIMS, 2011) .

On the other hand the stimulant effect of caffeine can counteract the relaxing effects of tranquillisers such as lorazepam. Patients being treated for anxiety with tranquillisers should not use caffeinated energy drinks or drink excessive quantities of coffee or tea (Mahan et al, 2011).

Warfarin and vitamin K-containing foods

Warfarin is a so-called anticoagulant which is used to thin the blood in patients with a tendency to increased clotting. It is used to treat patients after surgery who may be immobilised for lengthy periods, persons with deep vein thrombosis, or patients with heart disease linked to increased blood clotting to name but a few.

Warfarin acts by inhibiting the conversion of vitamin K into its active form and thus inhibiting the formation of 4 different vitamin K-dependent clotting factors. Vitamin K is found in green leafy vegetables (spinach, broccoli, asparagus, cabbage, green beans, lettuce) and avocado. St John’s wort may also counteract the effect of warfarin and patients on anticoagulant therapy may be advised by their doctors not to take herbal products which contain St John’s wort (Mahan et al, 2011).

It stands to reason that any patients using warfarin should check their nutrient supplements (e.g. vitamin, sports, and liquid meal supplements) for added vitamin K. If you need to take warfarin, always mention this when you purchase vitamin or sports supplements containing added vitamin K, and discuss the effect this vitamin may have on the efficacy of your anticoagulant treatment. Your pharmacist will be able to advise you.

Other foods that affect warfarin’s action

Warfarin can, however, also be affected by other foods or substances, which enhance the anticoagulant effect and may cause excessive bleeding (Mahan et al, 2011).

Onions, garlic, quinine (added to cold drinks such as tonic water), pawpaw, mango and vitamin E supplements (in doses higher than 400 IU per day) may all strengthen the effect of warfarin. According to Mahan and her coauthors (2011) the FDA has recently investigated the interaction between cranberries and warfarin. Despite a lack of evidence-based information, the FDA now requires cranberry products sold in the USA, to display a warning to this effect on their labels.

It is evident that a variety of foods are capable of affecting how certain medications work in the human body. Always remember to read the Patient Information Leaflet when you receive prescription medications and to take note if you are warned not to consume certain foods or drinks when using this treatment. You could just endanger your life or make your medication ineffective by eating or drinking the wrong foods or beverages together with your meds.

- (Dr IV van  Heerden, DietDoc, March 2012)                       


(Mahan LK et al (2011). Krause’s Food & the Nutrition Care Process. Ed. 13. Elsevier, USA. MIMS, (2011), Vol 51, No 10, October 2011.)

Any questions? Ask DietDoc

Read more:

Medication that affects nutrient intake
Why we don't take our meds 


Read Health24’s Comments Policy

Comment on this story
Comments have been closed for this article.