Living with Chronic Pain

What Are Drug Interactions?


A drug interaction occurs when a drug reacts with another drug or substance (e.g., food/beverage or supplement), changing how the medication works in the body. An interaction can also occur when a medication negatively impacts an existing medical condition. Drug interactions can increase or decrease the effectiveness of a medication and cause unexpected side effects.

Types of Drug Interactions

Drug interactions are categorized into four main types: drug-drug interaction, drug-food/drink interaction, drug-supplement interaction, and drug-condition interaction.

  • Drug-drug interaction
    A drug-drug interaction occurs when a medication reacts with another prescription or over-the-counter medication. For example, if the anticoagulant warfarin is taken with the antifungal medication fluconazole, dangerous bleeding can occur. If ibuprofen is taken with a diuretic, the ibuprofen can reduce the effectiveness of the diuretic.
  • Drug-food/drink interaction
    Certain foods or beverages can impact the way certain medications work. For example, eating grapefruit or drinking grapefruit juice while taking calcium channel blockers or statins can cause a dangerous accumulation of medication in the blood. Drinking alcohol while taking certain antidepressants can cause gastrointestinal bleeding.
  • Drug-supplement interaction
    Some dietary supplements can also change the way certain medications work. An example of this type of interaction is the combination of birth control pills and the herbal supplement St. John’s Wort, which can reduce the effectiveness of birth control pills. Another example is the combination of a blood thinner and a vitamin E supplement (which also thins the blood). This combination increases the risk of internal bleeding.
  • Drug-condition interaction
    A drug-condition interaction occurs when a medication negatively impacts an existing medical condition. A common example of a drug-condition interaction is when individuals with high blood pressure take a decongestant, which can increase blood pressure.

Just because a drug interaction can happen does not mean it will happen in all cases. Various factors, such as genetics, weight, age and gender, affect the risk of an interaction occurring. The dose of a medication and method of administration (oral, injection, topical, etc.) also play a part. Even seemingly minor factors, such as the order medications are taken or the amount of time between doses, can increase or decrease the likelihood of an interaction.

Individuals should keep their doctors and pharmacists informed about all prescription medications, over-the-counter medications, and supplements they take. It’s also important to inquire about any potential drug-food or drug-condition interactions of medications to help prevent drug interactions and any unwanted effects that could develop.

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