Drug interactions cause tens of thousands of hospitalizations every year in the US alone — and many are entirely preventable. The frustrating reality is that most people don't know which combinations to worry about, while pharmacists and doctors don't always have time to explain them fully.
Here are the ten most important drug interactions to be aware of, in plain English.
Check any combination of your medications instantly — free
Check interactions now →Ibuprofen doubles the bleeding risk for people on warfarin by both inhibiting platelet function and raising warfarin blood levels. This is one of the most dangerous common interactions. People on warfarin should use acetaminophen for pain instead. Read the full guide →
Taking antidepressants like Zoloft or Lexapro with ibuprofen or naproxen raises the risk of gastrointestinal bleeding by up to 15 times. Both drugs reduce platelet function through different mechanisms. This combination is extremely common and widely overlooked. Use Tylenol instead of ibuprofen if you're on an SSRI. Read the full guide →
Not all antibiotics interact with alcohol equally, but several — particularly metronidazole (Flagyl) and tinidazole — cause severe nausea, flushing, and heart palpitations when combined with even small amounts of alcohol. Others simply reduce effectiveness or worsen side effects. When prescribed an antibiotic, always ask whether alcohol is safe. Read the full guide →
Grapefruit and its juice contain compounds that block a critical liver enzyme (CYP3A4), causing many medications to build up to much higher levels than intended. Over 85 drugs are affected, including certain statins, calcium channel blockers, some antidepressants, and immunosuppressants. This interaction can persist for 24 hours or more after drinking grapefruit juice. Read the full guide →
ACE inhibitors like lisinopril and ibuprofen are a particularly risky combination for the kidneys. Both affect blood flow to the kidneys — and together, they can cause acute kidney injury, especially in elderly patients or those who are dehydrated. This combination also reduces the blood-pressure-lowering effect of lisinopril. Read the full guide →
Both Benadryl (diphenhydramine) and benzodiazepines depress the central nervous system. Combined, they can cause dangerous over-sedation, respiratory depression, and impaired coordination — especially in older adults. Many people don't realize Benadryl is in NyQuil and other sleep aids they take alongside anxiety medication. Read the full guide →
Heavy alcohol use with metformin raises the risk of a rare but serious condition called lactic acidosis. Additionally, both alcohol and metformin lower blood sugar, and together they can cause dangerous hypoglycemia. Occasional moderate drinking is generally acceptable, but heavy drinking is not safe on metformin. Read the full guide →
Omeprazole (Prilosec) blocks the liver enzyme needed to activate clopidogrel (Plavix), reducing the blood thinner's effectiveness by up to 50%. For heart patients relying on Plavix to prevent stent clotting, this is potentially life-threatening. Pantoprazole is a safer alternative PPI for patients on clopidogrel. Read the full guide →
Combining SSRIs (antidepressants like Prozac, Zoloft, Lexapro) with MAOIs (older antidepressants like phenelzine or tranylcypromine) can cause serotonin syndrome — a potentially fatal condition. There must be a washout period of at least 14 days between these medications. This interaction is well known among prescribers but still causes harm when patients see multiple doctors. Read the full guide →
Regular aspirin use (especially at 325mg doses) can reduce the blood-pressure-lowering effectiveness of ACE inhibitors like lisinopril by blocking a mechanism the drug relies on. For heart patients specifically prescribed both, doctors weigh this trade-off — but self-medicating with aspirin while on an ACE inhibitor should always be discussed with a doctor first. Read the full guide →
Check any combination of your medications — free and instant
Try MedCheck free →The best approaches are: (1) ask your pharmacist directly — they have access to your full medication history if you use one pharmacy; (2) use an online interaction checker like MedCheck; (3) ask your prescribing doctor when they add a new medication. Don't rely on the medication's package insert alone — it often uses complex language and may not account for your specific combination.
Not necessarily. Herbal supplements are pharmacologically active substances that can cause real drug interactions. Common problematic ones include St. John's Wort (interacts with many medications), ginkgo biloba (blood thinning), ginseng (blood sugar and blood pressure effects), and valerian (CNS depression). Always disclose all supplements to your pharmacist and doctor.
For mild interactions (reduced effectiveness), monitor for symptoms and inform your doctor at the next opportunity. For potentially serious interactions (bleeding risk, serotonin risk, severe sedation), contact your doctor or pharmacist immediately for guidance. For emergency symptoms (difficulty breathing, severe chest pain, signs of internal bleeding), call 911. When in doubt, call Poison Control: 1-800-222-1222.