What do I need to know about Drug Interactions?

Chemical reactions happen all around us every day. There are natural reactions that exist in the air, in our food, and even in our bodies to keep us healthy. When we take medications, this introduces a change to our body's natural function. When we take multiple medications, there is a potential that the drugs can interact with each other. Drug interactions can change our body's response for better or worse.

How does my body process medication?

The term pharmacokinetics describes what your body does to a drug. The commonly used abbreviation for the process is ADME which stands for Absorption, Distribution, Metabolism, and Excretion, which describes how a drug moves through and is processed by our body. Medications are absorbed into our system, distributed to their intended place of action, metabolized and activated or deactivated, and eliminated or removed from our bodies.

Enzymes in our bodies activate or deactivate medication (or substrates) in the metabolism stage to make them easier to eliminate. Drug interactions can impact this process by increasing or decreasing the amount of enzyme present.

How do drugs affect this process?

Drug-drug interactions can happen in a few different ways. One way is that drugs can change the acidity in your stomach which may affect the way medications are absorbed. An example of this is when we take antacids, like Tums, which may prevent some antivirals used to treat HIV, like dolutegravir, from being absorbed well, and reduce their effectiveness against the virus. A similar example is when certain vitamins or minerals that are present in a supplement bind with a drug and keep it from being absorbed. An example of this is iron supplements, which if taken with certain antibiotics, such as levofloxacin, are prevented from being completely absorbed, which means the antibiotic may not work as well.

Drug-drug interactions can also occur when drugs impact the way the enzymes work in your body. Drugs can bind to the enzymes and either stimulate them to metabolize medications faster or prevent them from metabolizing medications as well. An example of drugs that stimulate metabolism are anti-seizure medications like phenytoin or phenobarbital, while anti-fungal agents, such as fluconazole, block metabolism.

When medications induce enzymes, they may cause the body to get rid of the medication faster than expected making the medicine not work as well. When medications inhibit the enzymes, it may slow down the body's process to remove the drug, leading to higher-than-expected levels of the drug in the body. In some cases, this is done on purpose, but most of the time, this interaction could cause unwanted side effects.

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Can certain foods interact with drugs?

Yes! Foods can also affect how drugs work in similar ways to drug-drug interactions. Some examples of common food interactions include:

• Grapefruit and grapefruit juices. These inhibit enzymes and can block the metabolism of common medications, such as statins used to treat high cholesterol. It is recommended to avoid consuming grapefruit or grapefruit products with some statins.
• Calcium-rich foods and calcium-containing supplements. These can decrease the absorption of many medications including antibiotics and drugs used to treat high blood pressure. It is recommended to take all medications at least 2 hours before taking an antacid.
• Alcohol can have toxic interactions with several medications. Mixing alcohol with acetaminophen can produce a toxic product that damages the liver. Mixing alcohol with medications that alter a person's cognition can have an additive effect. Mixing alcohol with some antibiotics such as metronidazole can cause a disulfiram reaction that can cause nausea, dizziness, tachycardia ( a fast heartbeat), and other effects.

How can I prevent drug interactions?

• Pay close attention to the instructions given with your medication, and also to the warning labels on your medication bottles. They can offer insight into any potential interactions with foods or other medications.
• Notify your pharmacist and providers about all the medications you are taking (including prescription and over-the-counter (OTC) medications, herbals, or supplements) before starting or changing any medications. Keep a list of your medications, including prescriptions, OTC, and herbals, and bring the list to all visits with your provider.

How can my pharmacist help me prevent drug interactions?

Your pharmacist can check for drug interactions and advise you on how to handle them. They can also tell you how to monitor for efficacy changes or side effects that may occur as a result of a drug interaction.

Summary

Drug interactions can negatively affect your response to therapy. However, by being aware, keeping a list of ALL of your medications (OTC, prescription, vitamins, and herbal supplements), including those you take just “sometimes,” and making sure you communicate with your pharmacist or doctor, the risks can be minimized so that all of your medications can remain safe and effective.

See also:

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AHFS® Patient Medication Information is used with permission. ©2025, the American Society of Health-System Pharmacists, Inc. (ASHP). The ASHP Data is a part of the AHFS Drug Information®️; ASHP is not responsible for the accuracy of transpositions from the original context.