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Identifying Drug-Food Interactions

Edited by Adam Da Costa Gomes

Prescription and non-prescription medications can interact with each other and with foods, which can change the way they work in your body. Most drug interactions are not serious, but in some cases, these interactions can exacerbate or blunt the therapeutic intent of the drug, and commonly cause unintended side effects.

How Drug Interactions Occur

Drug interactions typically occur in two ways. First, interactions can occur pharmacodynamically when two agents act in a similar manner or affect the same receptor site. This can lead to a greater effect (if additive or synergistic) or a decreased effect (if antagonistic). Second, interactions can occur pharmacokinetically if one agent affects another drug’s absorption, distribution, metabolism, or excretion. Blood levels of a medication may be altered if taken with medications that affect metabolism. If drug metabolism increases, there is less medication in the body, which decreases the drug’s effectiveness. Vice versa, if drug metabolism decreases, there is more medication in the body, which increases the risk of side effects or toxicity.

It is crucial to be aware of all types of drug interactions because of their potentially lethal effects. Interactions can affect how the medication works by changing the amount of drug in your body, which can increase the risk of adverse drug effects and toxicity, or worsen existing medication conditions.

Knowing that interactions can occur pharmacodynamically or pharmacokinetically, it is now important to highlight which agents can interact with one another. There are three main types of drug interactions:

  • Drug-drug interactions are the most common type of drug interaction. The more medications in your therapy regimen, the higher the risk of an interaction between any two agents.

  • Drug-food/beverage interactions occur when a certain food or drink affects drug activity.

  • Drug-condition interactions happen when preexisting conditions affect the way a drug works in the body

Though all potential interactions are important to consider when taking any medications, we will focus on drug-food interactions in this article.

Common Drug-Food Interactions

Knowing what and how certain foods and beverages affect the way medications act in the body can help avoid unintended consequences. Listed below are some common drug-food interactions.

Statins and Grapefruit Juice

Many medications are metabolized by enzymes in the liver, which produce active and inactive metabolites depending on the medication. Statins are a class of medication used to lower cholesterol. If taken with grapefruit juice, furanocoumarin (a chemical found in grapefruits) can block enzymes in the liver that metabolize statins. Since these liver enzymes are no longer available to metabolize statins, this leads to a build-up of statins in the body and increases the risk of muscle pain and damage. Of note, this interaction only applies to atorvastatin (Lipitor), simvastatin (Zocor), and lovastatin (Mevacor). Rosuvastatin (Crestor), pravastatin (Pravachol), and fluvastatin (Lescol) are not metabolized by the aforementioned liver enzyme and can be a good alternative for those who regularly consume grapefruit juice.

Warfarin and Foods Rich in Vitamin K

Vitamin K is used in the production of clotting factors to prevent bleeding. For individuals who are at risk of blood clots, warfarin is a commonly used blood thinner that works by blocking enzymes in the liver to reduce the amount of vitamin K in the body. Consequently, consumption of vitamin K-rich foods such as kale, spinach, and other leafy greens decreases warfarin’s effectiveness, as they increase vitamin K levels in the body. Therefore, these foods should be eaten in consistent amounts to stabilize vitamin K levels in the body or, if possible, be avoided altogether by warfarin patients.

Monoamine Oxidase Inhibitors (MAOi) and Pickled, Cured, Fermented Foods

MAOis are a class of medications used to treat depression and includes phenylzine, tranylcypromine, isocarboxazid, and selegiline. They can interact with tyramine-containing foods such as aged cheese and cured meats by preventing the usual degradation of tyramine. A build-up of tyramine can lead to a significant increase in blood pressure. Though MAOis are an effective class of antidepressants, there are other drug classes that are safer and avoid this drug-food interaction.

Antibiotics and Dairy Products

Antibiotics such as tetracycline, doxycycline, and ciprofloxacin are widely prescribed to treat bacterial infections. If these are taken with dairy products (milk, yogurt, cheese, etc.), the calcium binds to the medication, which creates an insoluble substance that the body is unable to absorb. The decrease in absorption caused by ingesting calcium-rich products while taking an antibiotic can lead to a decrease in the effectiveness of the drug, meaning that the bacterial infection may not be entirely cleared.

Of note, not all drug interactions are bad. Some medications may be absorbed better in the body if taken with certain foods or beverages. For example, it is recommended to take iron supplements with a glass of orange juice because the vitamin C in orange juice may help increase iron absorption. On the other hand, some medications should be taken on an empty stomach. An example of this is levothyroxine, which should be taken 30 to 60 minutes before the first meal of the day because food decreases the drug’s absorption.

Other Factors that affect Drug Interaction

Other factors that may affect how drugs work in the body include the number of medications you take, age, kidney and liver function, diet, pre-existing medical conditions, and family history.

Alcohol, caffeine, and illicit drugs may also interact with medications. For example, it is advised against drinking alcohol while taking acetaminophen. Alcohol intake may affect the liver’s ability to function, which in turn, decreases the metabolism of acetaminophen and increases the risk of liver damage. Additionally, drinking alcohol while taking metronidazole, a commonly used antibiotic for urinary tract infections, may lead to facial flushing, vomiting, and increased heart rate. Those prescribed metronidazole are advised to avoid alcohol for the duration of treatment, plus 3 days after their last dose of metronidazole.

General Tips to Avoid Medication Problems

Major interactions are typically uncommon, but they can be serious and may lead to injury, hospitalization, or death. Many interactions found on medication labels are theoretical because they are based on the drug’s pharmacology. If you are concerned about a possible drug interaction, discuss the risks and benefits of taking the drug with your doctor. Switching to a similar medication with a different mechanism of action may help avoid potential drug interactions.

It is best to keep all medications in their original containers so you can identify them easier. The drug label or accompanying information sheet provides a plethora of important information, so read carefully! It includes what the medication is for, how to take it, and side effects to look out for. Additionally, keep an updated record of all your prescription drugs, over-the-counter (OTC) drugs, vitamins/supplements, and herbal medications. Bring this record to all of your medical appointments and notify your health care providers of any medication changes.

To reduce the chance of medication errors or duplicate therapy, pick up all of your medications at the same pharmacy. When you pick up a new medication (prescription or OTC), ask your pharmacist if you should avoid certain drugs, foods, or beverages while taking the new medication. Always consult your doctor or pharmacist before starting additional medications, including over-the-counter drugs. If you find a potential drug interaction, contact your doctor or pharmacist as soon as possible. Do not stop taking your medication without first consulting your healthcare provider.

Key Takeaways

Drug interactions occur when a substance affects how a drug works in the body. Certain foods and beverages may lead to pharmacokinetic or pharmacodynamic drug interactions. While these may increase the risk of toxicity or decrease the effectiveness of the medication, awareness of these drug interactions can help reduce the risk of these consequences. Always talk to your doctor before starting a new medication, whether it is prescription, over-the-counter, or a natural health product. Being proactive is the best way to prevent potential drug interactions.

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  1. Bushra R, Aslam N, Khan AY. Food-Drug Interactions. Oman Med J. 2011;26(2):77-83. doi:10.5001/omj.2011.21

  2. Drug Interactions Checker - For Drugs, Food & Alcohol. Accessed July 20, 2021.

  3. Hulisz D. Food–Drug Interactions. Accessed July 20, 2021.

  4. Johns Hopkins Medicine. Food-Drug Interactions. Accessed July 20, 2021.

  5. 5 Dangerous Food-Drug Interactions. Pharmacy Times. Accessed July 20, 2021.

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