Which Foods Interact With Your Blood Pressure Drugs?

food and drug interactions

Medicines used to lower blood pressure can interact with a surprising number of foods. The best known interaction is with grapefruit juice, but other foods can also affect the way your medicines work. In most cases, this results in slower metabolism of the drug so that blood levels increase. This may cause blood pressure to go too low, so you feel faint or dizzy, and also increases the risk of other side effects such as nausea or abdominal pain.

How food interacts with drugs

Most antihypertensive drugs are broken down by a large group of enzymes found in the gut and liver. Known as the cytochrome P450 family of enzymes, these also interact with a wide range of polyphenols and other substances found in plants. Some dietary substances stimulate increased production of these enzymes, while others block their effects.

Some grapefruit ingredients (eg naringenin, furanocoumarins) block the effects of an intestinal enzyme, CYP3A4, which metabolizes more than 60% of commonly prescribed drugs. As a result, the early breakdown of some drugs within the intestinal wall is reduced, so that more reaches the circulation. This blocking effect of grapefruit juice on CYP3A4 is irreversible, and the effects of the enzyme inactivation last for over 24 hours until you make new enzymes.

Grapefruit also appears to affect the transport of certain drugs across the intestinal wall, to slow their absorption, and to affect their metabolism in the liver. Taking one particular statin, lovastatin, with a glass of grapefruit juice produced the same blood levels of the drug as when taking 12 tablets with water!

grapefruit and drugs

The rate at which drugs are absorbed from the stomach or small intestines into the circulation is also affected by the presence or absence of food, and whether or not the drug is fat or water soluble. If fat soluble, for example, having a fatty meal will increase the drug’s absorption. These food effects can usually be avoided by taking a tablet up to 30 minutes before eating, but with some medicines this increases the risk of nausea. Drugs with a slow release formulation help to overcome these problems, by only slowly releasing the medicine into the intestines.

Always follow instructions in the patient information leaflet that comes with your medicine, as some antihypertensive drugs should be taken with food, and others on an empty stomach. If the leaflet doesn’t make any mentin of food or meals, then assume that there are no significant interactions with your particular formulation.

Food-Drugs Interaction Tool

There is a really useful drugs interaction tool at Drugs.com which includes potential food interactions. Type in the name of your drug and click the ‘Check for Interactions’ button. Then select the middle tab from the results, which will show any recognised alcohol/food interactions and provide excellent advice relating to that drug.

Interactions between food and diuretics

It’s usually best to take thiazide-like diuretics (eg chlortalidone) in the morning with food and a drink of water. You should avoid low salt diets. If you have problems passing water a lot at night, however, your doctor may change the way you take them. The diuretic indapamide does not seem to be affected when taken with food, however.

Taking some loop diuretics such as furosemide and bumetanide with a meal increases their absorption by as much as 30%. At the same time, food nutrients may compete for the attention of liver enzymes needed to break them down so that blood levels rapidly increase (eg hydrochlorothiazide.

Interactions between food and alpha blockers

Absorption of the alpha blocker, doxazosin, was found to increase by 31% in the presence of fatty foods. Extended release tablets can usual be taken with or without food, however. Other drugs, such as indoramin and prazosin have not been fully investigated for possible food-drug interactions.

Interactions between food and beta blockers

Some beta-blockers, such as metoprolol and timolol are not affected by food when taken in a sustained release formulation. Others, such as acebutolol and diacetolol are less well absorbed when taken with food. Metoprolol and propranolol uptake is increased by a high protein diet. Garlic also increases the uptake of propranolol although these effects do not seem to produce clinically important results on blood pressure control.

Grapefruit juice interacts with some beta-blockers by interfering with intestinal absorption.

Interactions between food and calcium-channel blockers

Food delays the absorption of some calcium channel blockers, such as felodipine, by increasing the time the drug stays in the stomach.

With verapamil, food increases absorption especially when taking it with protein rich foods. However, these effects are taken into account when deciding doses, and slow release preparations are usually preferably taken with or shortly after meals.

A low-fat, high carbohydrate meal slows the absorption of nifedipine but did not alter its effects.

Grapefruit juice interacts with most calcium channel blockers and the interaction was originally discovered by accident when researchers were looking at the effects of alcohol on felodipine absorption. Grapefruit juice was chosen as the mixer for its bitter taste, to disguise whether not alcohol was present. While alcohol did not have a significant effect on felodipine absorption, the grapefruit juice greatly increased blood levels of the medication.

Bitter Seville orange juice has a similar interaction with felodipine as grapefruit juice.

Effects of grapefruit juice on diltiazem are less pronounced.

Interactions between food and ACE inhibitors

No important food-drug interactions have been reported for angiotensin converting enzyme (ACE) inhibitors. Most can be taken before, with or after meals, as food intake does not affect their uptake or action.

Interactions between food and angiotensin II receptor blockers

Angiotensin II receptor blockers (ARBs) are rapidly absorbed but food can still slow their absorption – for example, by 40% to 50% for valsartan, 5% to 10% for losartan and 10% to 20% for telmisartan. Irbesartan is so rapidly absorbed that, generally, its effects are not affected by food.

Several ARBs are metabolized by cytochrome P450 so can interact with foods such as grapefruit juice. Blood levels of losartan, are less effected, but grapefruit juice slows its clearance from the body.

Interactions between food and hydrazine

Hydralazine is used in combination with other antihypertensive drugs when blood pressure is difficult to control. Taking hydralazine with food reduces blood levels by 69%, and even the slow-release form was reduced by 29%.

Check your patient information leaflet(s)

The way in which you respond to antihypertensive drugs is an individual thing, as different people inherit a different ‘blend’ of cytochrome P450 enzymes. As a result, some drugs will suit you and others won’t – either because you break them down more quickly or more slowly than other people. Similarly, the way certain foods affect your response to these drugs will vary.

Until everyone has easy access to DNA tests to predict their response to drugs, and their susceptibility to food-drug interactions, please follow the instructions provided with your blood pressure and other medicines carefully.

Image credit: handmade pictures/shutterstock

About DrSarahBrewer

Dr Sarah Brewer MSc (Nutr Med), MA (Cantab), MB, BChir, RNutr, MBANT, CNHC qualified from Cambridge University with degrees in Natural Sciences, Medicine and Surgery. After working in general practice, she gained a master's degree in nutritional medicine from the University of Surrey. Sarah is a registered Medical Doctor, a registered Nutritionist and a registered Nutritional Therapist. She is an award winning author of over 60 popular self-help books and a columnist for Prima magazine.

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