EVIDENCE

Which supplements interact with common medications?

The best-documented supplement and medicine interactions, from vitamin K and warfarin to St. John's wort, minerals and thyroid medicine, and 5-HTP with SSRIs.

The short version

Vitamin K works against warfarin, so people on warfarin need a steady vitamin K intake and should not start or stop a vitamin K supplement on their own.
St. John's wort can weaken many medicines, including birth control pills, warfarin, some heart medicines and transplant medicines.
Calcium and iron can reduce absorption of levothyroxine, and calcium and magnesium can bind some antibiotics, so spacing doses by a few hours matters.
Potassium supplements can push blood potassium too high in people taking ACE inhibitors, ARBs or potassium-sparing diuretics.
Red yeast rice acts like a statin, and 5-HTP adds to the serotonin effect of SSRIs and other mood medicines. Don't combine either without your prescriber.
If you take five or more medicines, ask a pharmacist for a full medication check before adding supplements.

Why do supplements interact with medicines?

Supplements interact with medicines in three main ways: they can block absorption in the gut, change how fast the liver clears a drug, or add to (or oppose) the drug's effect.

Most interactions are manageable once you know about them. The common fixes are spacing doses apart, keeping intake steady, or leaving one ingredient out. That's why it's worth bringing a full list of what you take, supplements included, to your doctor or pharmacist.

Vitamin K and warfarin

Warfarin works by opposing vitamin K, the vitamin your body uses to make clotting proteins. A sudden rise in vitamin K intake can weaken warfarin's effect, and a sudden drop can strengthen it.

The goal is consistency, not avoidance. People taking warfarin are advised to keep vitamin K intake from food and supplements about the same day to day. Don't start or stop a product containing vitamin K, including a multivitamin, without telling the clinician who manages your warfarin.

Apiece's safety rules leave vitamin K out of a formula when someone reports taking warfarin.

St. John's wort: one herb, many interactions

St. John's wort, an herb often taken for low mood, has more documented interactions than almost any other supplement. It can weaken the effect of many medicines, including:

Birth control pills
Warfarin and other blood thinners
Some heart medicines, such as digoxin
Cyclosporine, used after organ transplants
Some HIV medicines

It also adds to the effect of SSRIs and other medicines that raise serotonin, which can lead to serious serotonin-related side effects. If you take any prescription medicine, check with a pharmacist before using St. John's wort.

Calcium, iron and magnesium: spacing from thyroid medicine and antibiotics

Minerals can bind some medicines in the gut so less is absorbed. Spacing doses solves most of this.

Levothyroxine (thyroid hormone): the drug's label tells people taking calcium carbonate to keep the two at least 4 hours apart. Iron supplements, coffee, soy and fiber also reduce levothyroxine absorption.
Quinolone antibiotics, such as ciprofloxacin: calcium lowers absorption, so take the antibiotic at least 2 hours before or after a calcium supplement.
Tetracycline antibiotics, such as doxycycline, and quinolones: magnesium forms complexes with both, so follow the spacing on your prescription.
Oral bisphosphonates for bone: take magnesium-rich supplements or medicines at least 2 hours before or after.

If you take one of these medicines in the morning, taking your supplements later in the day is often the simplest fix. Your pharmacist can confirm the right gap for your specific medicine.

Potassium and some blood pressure medicines

Potassium supplements can raise blood potassium too high in people taking ACE inhibitors, angiotensin receptor blockers (ARBs) or potassium-sparing diuretics, because these medicines reduce how much potassium leaves in urine. The risk is greater with kidney problems.

Salt substitutes are often potassium chloride, so they count too. If you take one of these medicines, ask your doctor how much potassium is safe for you from food, drinks and supplements combined.

Grapefruit-style effects: how the liver clears drugs

Grapefruit juice is the classic example of an enzyme interaction. It blocks CYP3A4, an enzyme in the gut that normally breaks down some drugs, so more drug reaches the bloodstream and stays longer. Some statins, blood pressure medicines and anti-anxiety medicines are affected.

Some supplements work the same way in general terms: they speed up or slow down drug-clearing enzymes. St. John's wort is the best-known herb in this group, which is why it weakens so many medicines. Because these effects vary by drug, your pharmacist is the right person to ask.

Metformin and vitamin B12

Long-term metformin use is linked to lower vitamin B12. In a large US study, borderline-low B12 was about twice as common with metformin as with placebo after 5 years (19.1% vs 9.5%), and each year of metformin use raised the odds of low B12.

The study authors suggested routine B12 testing for people taking metformin. If you take it, ask your doctor about checking B12. Apiece's daily foundation includes vitamin B12 for everyone.

Statins and red yeast rice; SSRIs and 5-HTP

Red yeast rice can contain monacolin K, which is chemically identical to the statin lovastatin. That means it can have the same side effects as statins, including muscle, kidney and liver problems, and the same drug interactions. Taking it alongside a prescription statin doubles up on the same mechanism. Amounts also vary widely: a 2017 review of 26 products found monacolin K content differed more than 60-fold.

5-HTP is a building block the body uses to make serotonin. Combining it with SSRIs or other serotonin-raising mood medicines can push serotonin too high, a serious reaction called serotonin syndrome. A published case report describes this reaction in a man taking 5-HTP with the SSRI sertraline. Don't combine them without your prescriber's approval.

When should you ask a pharmacist?

Ask before adding any supplement if you take a prescription medicine, are pregnant, or have a health condition. If you take five or more medicines, ask your pharmacist for a full medication check that includes your supplements.

Bring the actual bottles or a complete list with amounts. A product that lists every ingredient and amount, with no proprietary blends, makes this much easier.

Questions

Can I take vitamin K if I'm on warfarin?

Only with your clinician's guidance. Warfarin opposes vitamin K, so sudden changes in vitamin K intake can make warfarin weaker or stronger. People on warfarin are advised to keep vitamin K intake steady and not start or stop a vitamin K supplement on their own.

How long should I wait between levothyroxine and calcium or iron?

The levothyroxine label advises at least 4 hours between it and calcium carbonate supplements, and iron supplements also reduce its absorption. Many people take thyroid medicine in the morning and supplements later in the day. Confirm the timing with your pharmacist.

Does St. John's wort interact with birth control?

Yes. St. John's wort can weaken the effect of birth control pills, as well as warfarin, some heart medicines, transplant medicines and some HIV medicines. Talk to your doctor or pharmacist before using it with any prescription.

Is it safe to take 5-HTP with an SSRI?

It's not recommended without your prescriber's approval. 5-HTP is used by the body to make serotonin, and combining it with SSRIs or other serotonin-raising mood medicines can lead to serotonin syndrome, a serious reaction.

Does metformin lower vitamin B12?

Long-term use is linked to lower B12. In a large US study, borderline-low B12 was about twice as common with metformin as with placebo after 5 years. Ask your doctor about checking your B12 if you take metformin.

Can I take red yeast rice with a statin?

Not without your doctor's input. Red yeast rice can contain monacolin K, which is identical to lovastatin, so it can add to a statin's effects and side effects, including muscle, kidney and liver problems.

Sources

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*These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease. This page is general information, not medical advice. Check with your doctor or pharmacist before starting a supplement, especially if you take medication, are pregnant or nursing, or have a health condition. Updated 2026-10-20.