Supplements can interact with medications in ways that reduce drug effectiveness or increase side effects. Here are the most important interactions to know.
Major Interactions (Always Check)
St. John's Wort Interacts with MANY drugs: birth control, antidepressants, blood thinners, HIV medications, immunosuppressants, some cancer drugs. Speeds up drug metabolism, making medications less effective.
Vitamin K Affects blood thinners (warfarin). Consistent intake is okay; sudden changes cause problems. If on warfarin, don't start or stop K supplements without doctor guidance.
Fish Oil / Omega-3s May increase bleeding risk with blood thinners (warfarin, aspirin, clopidogrel). Usually safe at moderate doses but discuss with doctor.
Grapefruit / Bergamot Extract Affects the same liver enzyme (CYP3A4) as grapefruit juice. Can increase blood levels of statins, some blood pressure meds, and others.
Moderate Interactions
Magnesium and Zinc Can reduce absorption of some antibiotics (fluoroquinolones, tetracyclines) and bisphosphonates. Take 2+ hours apart.
Calcium Same as magnesium—can bind to certain medications. Separate by 2+ hours.
Iron Reduces absorption of levothyroxine (thyroid medication). Take 4 hours apart.
Berberine Acts like metformin. If you're on diabetes medication, blood sugar could drop too low.
CoQ10 May reduce effectiveness of blood thinners. May enhance blood pressure medication effects.
Melatonin May enhance sedation with sleep medications or anti-anxiety drugs.
How to Check Interactions
Online databases: - drugs.com interaction checker - WebMD interaction checker - Natural Medicines database (subscription)
Ask your pharmacist: Pharmacists are trained in drug interactions. They can check supplements against your medication list.
Tell your doctor: Many doctors don't ask about supplements. You need to volunteer this information, especially before surgery or starting new medications.
Our stack analyzer: We check for common interactions when you enter your medications. But this doesn't replace professional advice.
General Guidelines
1. Time separation helps: Many interactions involve absorption. Taking supplements 2+ hours away from medications often solves the problem.
2. Consistency matters more than avoidance: With vitamin K and warfarin, the issue isn't eating greens—it's changing your intake suddenly. Consistent intake allows dose adjustment.
3. "Natural" doesn't mean "safe with drugs": Herbal supplements are the biggest interaction risk. St. John's Wort, kava, ginkgo, garlic, and ginseng all have notable interactions.
4. Report new symptoms: If you start a new supplement and notice changes in how you feel or how your medication works, tell your doctor.
The Bottom Line
Most common supplements (vitamins, minerals, fish oil at normal doses) are safe with most medications. But interactions exist, and some are serious. When in doubt, check with a pharmacist or doctor.