A drug interaction happens when a substance affects how a drug behaves in the body. Some substances that can cause interactions include other drugs, supplements, food, and alcohol.

Drug interactions can make a drug less effective. In some cases, they can make a drug more potent, which can be dangerous.

The symptoms of drug interactions vary greatly and range from mild to life threatening. Some common warning signs include feeling sick after taking a medication, not getting the usual results from a medication, or feeling either very tired or very energetic after taking a medication.

Drug interactions do not only occur between prescription drugs. Over-the-counter drugs, food, supplements, and alcohol can all change the way the body metabolizes (breaks down) medication.

Read more to learn about different types of drug interactions, risk factors, and how to help prevent interactions by thoroughly reading drug labels.

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There are several types of drug interactions.


Drug-drug interactions occur when one drug interacts with another.

Certain drugs are more prone to interactions than others. For example, warfarin is an anticoagulant used to help prevent blood clots. It interacts with many other medications.

This is because a group of enzymes (called CYP450 enzymes) change the way the body metabolizes warfarin. Drugs that inhibit these enzymes can increase the effects of warfarin, increasing the risk of dangerous bleeding. Drugs that induce the enzymes reduce warfarin’s effectiveness, raising the risk of blood clots.

These potentially dangerous health outcomes show how important it is to discuss new medications with a doctor or pharmacist.

Safe disposal of medication

Inappropriately discarded drugs can harm people, animals, and the environment. It is essential to dispose of any unwanted medication safely. Read our guide on medication disposal here.

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A 2019 poll from the American Osteopathic Association found that 86% of Americans take vitamins or supplements.

While supplements are beneficial for people with a deficiency, they still have potential side effects and interactions, just like any other medication. The supplements a person takes can interact with prescription drugs or other supplements.

For example, calcium can affect the absorption of certain drugs.

This includes both calcium supplements and calcium-containing drugs, such as certain antacids. Calcium can also affect the body’s ability to absorb the thyroid drug levothyroxine, which treats hypothyroidism.

When discussing medication with a doctor, a person should be sure to note any vitamins, supplements, and herbal remedies they take.

Further resources

For more in-depth resources about vitamins, minerals, and supplements, visit our dedicated hub.

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Alcohol and other recreational drugs

Alcohol and recreational drugs interact with many different drugs. They have the potential to interact with prescription drugs, supplements, and other recreational drugs.

Alcohol can interact with alcohol-containing drugs, such as cough syrup and laxatives. Consuming alcohol while taking these medications can intensify the effects of alcohol, which can be harmful. It may also interact with other drugs that slow activity in the central nervous system, making a person sleepy. Some of these drugs include:

Recreational drugs can also cause drug interactions. When a person takes cannabis with certain antipsychotic drugs, such as clozapine, it reduces the amount of the antipsychotic in their bloodstream. This makes the drug less effective, which can be dangerous. The commonly prescribed antidepressant citalopram (Celexa) can cause brain bleeding when taken with cocaine.

Individuals with mental health conditions are more likely to have a substance abuse disorder. This means that the drug they are taking to treat their mental health condition may not be as effective if they also have a substance abuse disorder. If an individual or loved one is concerned about mental illness and substance abuse, they should contact a doctor or one of the organizations below:

Help is available

Seeking help for addiction may feel daunting or even scary, but several organizations can provide support.

If you believe that you or someone close to you is showing signs of addiction, you can contact the following organizations for immediate help and advice:

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Certain foods affect how the body metabolizes certain drugs.

One common culprit is grapefruit. An enzyme called CYP3A4 helps metabolize many drugs in the small intestine, but grapefruit blocks this enzyme. This makes more of the drug enter the bloodstream, resulting in a higher dose that stays in the body longer.

CYP3A4 levels vary from person to person, so it is difficult to predict how grapefruit might affect an individual. Some people might have no side effects at all. Others may experience life threatening drug toxicity.

Some medications that interact with grapefruit include:

Learn more about interactions between grapefruit and statins, which are used to lower cholesterol and treat heart disease.

Medical conditions

Certain medical conditions can make a person more likely to experience a drug interaction.

For example, people with high blood pressure may want to avoid certain decongestants. Some of these, such as pseudoephedrine (found in many cold remedies), may increase blood pressure, making them an unsafe choice.

Additionally, people with dementia are highly vulnerable to a number of drug interactions. One 2016 cross-sectional study found that antidepressants, antipsychotics, antiplatelets, and omeprazole were all more likely to cause interactions in older people with dementia. Frequently prescribed dementia medication was not a contributor to severe drug interactions.

People with certain medical conditions, especially ones requiring a person to take medication regularly, should contact a doctor before starting any new medications. This reduces the risk of a potentially harmful drug interaction.

The more drugs (including supplements) a person takes, the higher their risk for drug interactions.

People taking many different medications should read the label for each drug. They should also consult a doctor or pharmacist about any potential interactions.

Some people are more vulnerable to drug interactions than others, regardless of the number of drugs they take. Factors that increase a person’s risk for drug interactions include:

Thoroughly reading drug labels can help prevent drug interactions. If an individual is confused or concerned about a potential interaction, they should contact a doctor.

Supplement and prescription drug labels are different.

Prescription drugs must list known drug interactions. Drug labels usually list the most severe and common interactions. They may also have an insert with a heading that says, “Drug Interactions.” A person should read this section thoroughly.

Supplement drug labels must be accurate and not misleading. But there is no requirement for them to list all drug interactions. People taking supplements should tell their providers about all supplements they take.

One of the best ways to reduce the risk of drug interactions is to talk with a doctor before taking a new drug, including supplements.

Contact a doctor if:

  • a new medication is not working or is producing an unexpected result
  • a person becomes sick or develops any unusual symptoms, such as sleepiness
  • a person wants to try a new supplement

Go to the emergency room if:

  • a person shows signs of a severe reaction, such as a rash, trouble breathing, or lethargy
  • a person takes a dangerously high dose of a drug
  • a child consumes supplements or prescription medications

Any food or substance can interact with any other substance. In some cases, these reactions can be very dangerous.

The best way to avoid drug interactions is to take only drugs that are needed and read all new drug labels thoroughly.

If a person is unsure or is concerned about a potential drug interaction, they should contact a doctor or pharmacist.

When someone picks up a new prescription, they should ask about common drug interactions and tell the pharmacist about all other medications and supplements they are taking.

People who have negative drug reactions should talk with a doctor right away.