Food and Medication Interactions You Should Know About

| Posted On Oct 26, 2016 | By:

food-and-drug-interaction-shutterstock_275841548When I prescribe a new medication for one of my patients or suggest an over-the-counter (OTC) drug, I like to spend some time with them to explain the medication, how it works, possible side effects, and how to take it. A big part of taking a medication involves knowing what you can or can’t take with it, because combining certain foods with certain medications can have an important effect on how the drug works in your system.

If you’re taking a medication regularly and are not clear on how it should be taken – on an empty stomach or with a specific food – or what foods may affect how it works, you should always ask your clinician or pharmacist.

Below I’ve outlined some common medications and what you should know about their interactions with certain foods.

Grapefruit juice is particularly reactive with anti-hypertensive medications known as calcium channel blockers. The juice enhances the effects of those drugs, and your body therefore receives more of the drug than would otherwise be typical. As these drugs are meant to slow down the heart rate, this can be very dangerous. You should not drink grapefruit juice at the same time that you take your medicine. If you do drink grapefruit juice, discuss it with your clinician so that he or she can instruct you as to when during the day it is safe to drink and can be sure that your calcium channel blocker is dosed appropriately.

Warfarin (Coumadin®) and other “blood thinners”

These are medications used to treat blood clots or prevent a stroke after someone is in atrial fibrillation. The medication requires close monitoring because there is a “narrow therapeutic window” to act if a patient takes too little (and the blood is not thin enough) or too much (and the blood is too thin to clot well). Changes in the amount and types of food in your diet can have a major impact on the action of this drug. For example, having a stomach bug can completely throw off someone’s warfarin levels, leading to blood which is too thin, putting a person at risk for bleeding.

In particular, though, eating foods with vitamin K have a big impact, as warfarin and vitamin K work against each other. What foods contain vitamin K? Vitamin K rich foods are green leafy vegetables like kale, parsley, spinach, and Brussel sprouts. Any change in the amount of these foods you eat can throw off levels quite quickly and dramatically. Soy and cranberry juice also have some effect, but interact differently. Cranberry juice, for example, changes how the drug is metabolized by the liver.

When people have too much warfarin in their system, we actually give them vitamin K to reverse the medication. Therefore, anyone who is taking warfarin needs to maintain a very steady intake of vitamin K rich foods. Your provider will dose your warfarin taking into account your diet. If people are diligent and consistent with their diets, they typically need fewer blood draws to monitor coagulation level(regardless, no more than 4 weeks between draws is recommended).

MAO Inhibitors (MAOIs)

Although rarely prescribed these days, these anti-depressant drugs have significant dietary interactions. When we do encounter a patient who still uses an MAO inhibitor, we counsel to completely avoid any food that contains tyramine: bananas, yogurt, red wine, mature cheeses, and cured meats. People who take MAO inhibitors can’t break down tyramine properly. As a result, too much tyramine can enter the bloodstream and raise blood pressure, leading to a hypertensive crisis that is very dangerous.

Note: If you had been on an MAO inhibitor and successfully switched to a serotonin reuptake inhibitor (SSRI), these food concerns do not apply, as SSRIs are a completely different type of drug.

Thyroid Medications

These are best taken first thing in the morning on an empty stomach. It’s also best to wait an hour before eating or drinking anything. Grapefruit juice interacts with levothyroxine, a common thyroid medicine, and while not dangerous per se, it does delay its absorption and could affect the dose of the drug you receive. Calcium binds up the drug and it therefore cannot be absorbed into the bloodstream, so it’s best to avoid calcium (calcium pills or chewable Tums® tablets) for at least two hours before or after taking your thyroid medications.


Some antibiotics’ effectiveness can be completely neutralized if you take them with certain foods. For example, milk can bind up certain antibiotics and not allow the drug to be absorbed and work properly.

It is recommended that take antibiotics with a full glass (6oz. or more) of water, not just a sip of water. Some medicines, if not moved quickly through the esophagus, can cause irritation and damage to the lining. A good example is doxycycline, a common antibiotic used to treat Lyme disease, pneumonia, and acne.

And a few additional comments on some foods to be aware of if you take certain prescription or OTC medications:

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About Dr. Michele Coviello

Dr. Coviello joined Atrius Health in 2016 and is the Chief of Internal Medicine at our Harvard Vanguard - Needham practice. She received her medical degree from Tufts University School of Medicine in Boston and completed both her internship and residency at New England Deaconess Hospital in Boston.