When you prescribe a medication to patients, you expect it to help them. Maybe it handles their high blood pressure, reduces their cholesterol, or eases chronic pain they may be facing. But, as a pharmacist, you also expect that you may face dangers.

Enter the world of drug interactions.

In pharmacy school, you probably learned common drug interactions to avoid: digoxin and quinidine, clonidine and propranolol, methotrexate and probenecid — the list goes on. Put together, these medications can cause a variety of adverse drug reactions.

In fact, the National Institutes of Health estimates that over 2 million people are hospitalized for adverse drug reactions (ADRs) every year. Of those hospitalized, over 100,000 people die.

These estimates suggest that ADRs are the 4th leading cause of death in the United States, ahead of pulmonary disease, diabetes, accidents, and even automobile deaths.

Clearly, drug interactions are a serious concern for pharmacists everywhere. Likely, you already take steps to prevent drug interactions — but when’s the last time you took a close look at the plan you have in place?

Read on to get up to date on all things drug interactions, refresh your response plan, and keep your patients safe.

Types of Drug Interactions

To recap from pharmacy school, there are three types of drug interactions:

  • Drug-drug interaction: A reaction between two (or more) drugs.
  • Drug-food interaction: A reaction between a drug and a food or beverage.
  • Drug-condition interaction: A reaction that occurs when taking a drug while having a certain medical condition; for example, taking a nasal decongestant with high blood pressure.

Drug interactions can happen for many reasons. For one, many medications change the way that liver enzymes, called cytochrome P450 (CYP450) enzymes, work in the body. CYP450 enzymes are responsible for activating or breaking down many medications and then removing them from the body.

When these enzymes are disrupted (by foods, drugs, or comorbidities), they can cause a medication to stay too long in the body, build up to toxic levels, or, on the other hand, not stay in the body long enough.

Aside from CYP450 enzymes, many medications are affected by absorption — which can cause adverse drug interactions. Some medications may bind to food, dietary supplements, or tissues in the body, which stops drugs from entering the bloodstream and making them less effective.

Common Drug Interactions

According to GoodRx, some of the most common drug interactions include:

Drug-Drug Interactions

  • Fluconazole and simvastatin: Fluconazole (Diflucan) can increase blood levels of simvastatin (Zocor), a cholesterol medication. This may lead to higher risks of simvastatin side effects, like abdominal pain, bloating, and rash.
  • Ondansetron and dofetilide: Ondansetron (Zofran) and dofetilide (Tikosyn) can both prolong the time between heartbeats. When used together, this can cause dizziness, fainting, or in severe cases, death.
  • Bactrim and warfarin: Bactrim (sulfamethoxazole/trimethoprim), an antibiotic, can increase the risk of bleeding when used with the blood thinner warfarin (Coumadin, Jantoven), potentially leading to serious bleeding complications.

Drug-Food Interactions

  • Atorvastatin and grapefruit: Grapefruit and grapefruit juice can increase the amount of atorvastatin (Lipitor), a cholesterol medication, in the body by blocking certain CYP450 enzymes. This increases the risk of side effects like headaches, nausea, and cold-like symptoms.
  • Metformin and alcohol: Metformin, a diabetes medication, and alcohol both can elevate lactic acid levels in the bloodstream, potentially causing lactic acidosis.
  • MAOIs and meat: MAOIs, like Emsam and Nardil, inhibit the breakdown of tyramine, a protein found in aged meats like sausage and salami. Excessive tyramine intake can lead to a sudden increase in blood pressure.

Drug-Condition Interactions

  • Propranolol and asthma: Propranolol, a beta blocker for conditions like high blood pressure, can tighten the muscles aiding breathing, potentially triggering asthma attacks.
  • Diphenhydramine and glaucoma: Diphenhydramine (Benadryl), an antihistamine, can elevate intraocular pressure, making it unsuitable for patients with glaucoma.
  • Acetaminophen and liver disease: Acetaminophen (Tylenol), while generally safe, can be harmful for patients with liver disease. Accumulation of acetaminophen can lead to liver damage and cause adverse reactions.

8 Tips for Preventing Drug Interactions

Drug interactions come in many shapes and sizes, but they should all be treated with caution. If you’re looking to refresh your plan for preventing drug interactions, here are 8 steps to take:

  1. Stay informed: Stay up to date on the latest drug information, including potential interactions, by regularly reviewing drug databases, clinical guidelines, and relevant literature. A few resources you may want to consult include Medscape’s Drug Interaction Checker, Certara’s Drug Interaction Database (DIDB), and the
  1. Perform medication reviews: Conduct thorough medication reviews for each patient to identify potential interactions. In your review, you should consider factors like the patient’s medical history, current medication regimen, allergies, and lifestyle factors.
  1. Use technology: In the fight against drug interactions, technology can help. This is especially true in the case of your pharmacy software system. Many systems have built-in tools to help you prevent drug interactions, like queues and prescription verifications. If you find that your software system doesn’t have the tools to meet this need, consider switching to a new one.
  1. Assess risk factors: Before you prescribe a new medication, consider patient-specific risk factors that may predispose them to experiencing drug interactions — like age, comorbidities, and genetics. Some software systems also include built-in risk assessment tools to help you minimize adverse reactions.
  1. Educate patients: It’s not just you who should stay informed — but also your patients. Provide comprehensive counseling to patients about their medications, including potential interactions, side effects, and proper usage instructions. At the end of your session, encourage patients to ask questions and report any adverse effects they experience. If they do, adjust their regimen accordingly.
  1. Promote communication: On the note of communication, encourage open communication between patients, healthcare providers, and pharmacists regarding all medications the patient is taking, including prescription medications, over-the-counter drugs, vitamins, and supplements. This can also be a great way to strengthen your relationships with other providers and be a more effective healthcare team.
  1. Offer alternatives: For patients facing a particular medication risk, recommend alternative medications or therapeutic approaches. This can minimize interactions without sacrificing the patient’s health.
  1. Document interactions: If a patient does happen to have a drug interaction, document those interactions, the interventions made to address them, and any recommendations given to providers or prescribers. Once again, your pharmacy software system should enable you to document interactions seamlessly — and prevent them from happening again.


While many medications can be helpful, they aren’t without their harms.

On the bright side, by educating yourself, working with patients and providers, and considering the best methods of action for your patients, you can minimize drug interactions and prevent adverse reactions.

In the process, you can ensure that your patients get the treatment they need without compromising their health. That’s all a pharmacist can hope for.