“Can I Take This With My Other Medicine?”: Understanding Drug Interactions

The pharmacist serves as the final safety check by ensuring that a medication is not only correctly filled but is also clinically safe for the patient. Ideally, pharmacists perform a structured review of the patient’s full medication regimen, including OTC medications and supplements, prescription medications, and known allergies and health conditions, during which they can identify any drug interactions.

These interactions can decrease a drug’s effectiveness, cause drug toxicity, and/or lead to unexpected adverse effects. Drug interactions are not isolated to other drugs but may also occur with foods, drinks, and health conditions.¹

Drug-Drug Interactions

Drug-drug interactions occur when 2 or more medications interact with one another. Taking 2 or more medications with additive or synergistic effects, such as a sedative and an antihistamine, can amplify the effects, which can then become toxic. In this example, this may result in dangerous levels of drowsiness.²

Alternatively, if 2 or more drugs have opposing effects, one drug may counteract the benefits of another. For example, corticosteroids may oppose the blood glucose–lowering effects of some antidiabetic agents, requiring an increase in the dose of antidiabetic medication.²

Finally, one drug may alter the metabolism of another, resulting in toxic levels or increased clearance of the other. For example, some selective serotonin reuptake inhibitors may slow the metabolism of some anticonvulsants, leading to toxicity.²

Drug-Food and -Beverage Interactions

Certain foods and beverages can also change the way drugs are processed in the body, reducing a drug’s effectiveness or increasing its toxicity. Common mechanisms can include absorption interference (eg, dairy products and certain antibiotics), enzyme inhibition (eg, grapefruit juice and CYP3A4), direct counteraction (eg, vitamin K and blood thinners), and chemical reactions (eg, tyramine in aged cheeses, processed meats, and red wine with monoamine oxidase inhibitors).^3,4

Drug-Condition Interactions

Also known as drug-disease interactions, these occur when a drug taken to treat one condition negatively impacts another preexisting condition. Although these interactions can affect anyone, they are most common among older adults, patients using polypharmacy, and patients who use OTC medications. Some examples include a patient with hypertension using a nasal decongestant and a patient with liver disease using acetaminophen.¹

The Science Behind Interactions

Drug interactions can either change the concentration of the drug in the body through pharmacokinetics or the body’s response to that drug through pharmacodynamics. Pharmacokinetic interactions occur when a drug, food, beverage, or condition alters the absorption, distribution, metabolism, or excretion of another drug. This results in a change in the concentration of the affected drug. Pharmacodynamic interactions occur when a drug, food, beverage, or condition acts on the same physiological system or receptor as another drug. This changes the drug’s clinical effects without changing the drug’s concentration.

High-Risk Categories

Drugs with a narrow therapeutic index have a very small margin between a dose that is safe and effective and one that is toxic or ineffective. Minor interactions can result in severe adverse effects or treatment failure. Examples include warfarin, digoxin, lithium, and theophylline.⁵

Approximately 70% to 90% of all clinically used medications rely on the delicate balance of the CYP450 system in the liver for metabolism. When a substance changes the activity of these essential liver enzymes, a drug can become toxic or ineffective. Examples include grapefruit juice, azole antifungals, and St John’s wort.^6,7

OTC products often contain active ingredients that overlap with prescription medications or interfere with how the body processes prescription medications. Examples include nonsteroidal anti-inflammatory drugs that increase bleeding with blood thinners, antacids that decrease absorption of some medications, and St John’s wort, which can significantly reduce the effectiveness of hormonal birth control.⁸

How Pharmacists Protect Patients

As the final safety barrier, pharmacists use their expertise coupled with advanced technology to identify, prevent, and manage potential drug interactions. Every time a prescription is filled, pharmacy management systems software scans the patient’s medication profile. The pharmacist will receive real-time alerts when a potential interaction is identified, along with its severity level.

Additionally, a drug utilization review is performed each time a prescription is filled. This is a structured, ongoing program that collects, analyzes, and interprets drug use patterns and screens for many issues, including drug interactions. During patient counseling, communicate which foods and drinks the patient should avoid while taking medication, and use this time to screen for OTC and/or supplement consumption.

When an interaction is flagged as potentially dangerous, pharmacists act as a bridge between the patient and the prescriber. Relay the appropriate information and suggest a dose adjustment or alternative medication that will not result in a potentially dangerous interaction.

About the Author

Kathleen Kenny, PharmD, RPh, earned her doctoral degree from the University of Colorado Health Sciences Center in Aurora. She has more than 30 years of experience as a community pharmacist and works as a clinical medical writer based in Albuquerque, New Mexico.

REFERENCES

1. What is a drug interaction? Hivinfo.nih.gov. Updated March 31, 2025. Accessed June 16, 2026. https://hivinfo.nih.gov/understanding-hiv/fact-sheets/what-drug-interaction

2. Understanding drug interactions. Specialist Pharmacy Service. Updated May 22, 2024. Accessed June 16, 2026. https://www.sps.nhs.uk/articles/understanding-drug-interactions/

3. Grapefruit juice and some drugs don’t mix. FDA. Updated July 1, 2021. Accessed June 16, 2026. https://www.fda.gov/consumers/consumer-updates/grapefruit-juice-and-some-drugs-dont-mix

4. Medication interactions: food, supplements and other drugs. American Heart Association. Reviewed January 18, 2024. Accessed June 16, 2026. https://www.heart.org/en/health-topics/consumer-healthcare/medication-information/medication-interactions-food-supplements-and-other-drugs

5. Alahmari A, Fatani S, Ahmed N. Drug-drug interactions: a descriptive analysis of FDA adverse event reporting system. Medicine (Baltimore). 2025;104(38):e44606. doi:10.1097/MD.0000000000044606

6. Patel T, Rahimi N, Cassagnol M. Biochemistry, cytochrome P450. In: StatPearls. StatPearls Publishing; 2026. Accessed June 16, 2026. https://www.ncbi.nlm.nih.gov/books/NBK557698/

7. For healthcare professionals | FDA’s examples of drugs that interact with CYP enzymes and transporter systems. FDA. May 29, 2026. Accessed June 16, 2026. https://www.fda.gov/drugs/drug-interactions-labeling/healthcare-professionals-fdas-examples-drugs-interact-cyp-enzymes-and-transporter-systems

8. St John’s wort. Mayo Clinic. March 21, 2025. Accessed June 16, 2026. https://www.mayoclinic.org/drugs-supplements-st-johns-wort/art-20362212

9. Thapa RB, Karki S, Shrestha S. Exploring potential drug-drug interactions in discharge prescriptions: ChatGPT’s effectiveness in assessing those interactions. Explor Res Clin Social Pharm. 2025;17:100564. doi:10.1016/j.rcsop.2025.100564