Some supplements stop medicines from being absorbed, some change how fast your liver clears a drug, and some add to a medicine’s effect, such as bleeding. The first kind can often be fixed by taking things a few hours apart; the other two can’t, so the safest habit is to tell your doctor and pharmacist about every vitamin, herb and “natural” product you take.
Below are the combinations that cause the most trouble, with what to do about each.
Three ways supplements and medicines interact
- Blocking absorption in the gut. Minerals like calcium, iron, zinc and magnesium can bind to certain medicines so less gets into your blood. Spacing doses usually solves this.
- Changing how your body processes a drug. Some herbs speed up or slow down the liver enzymes that break medicines down, so the drug level in your blood falls or rises. Spacing doesn’t help, because the effect lasts all day.
- Adding to a drug’s effect. If a supplement and a medicine both affect clotting, or both lower blood sugar, their effects stack. Again, spacing doesn’t help.
Minerals that block absorption: calcium, iron and zinc
These are the interactions you can manage with timing. Your pharmacist can confirm the gap for your exact medicine, but these are the standard ones:
| Medicine | Clashes with | Usual advice |
|---|---|---|
| Levothyroxine (thyroid tablet) | Calcium carbonate, iron (ferrous sulfate), antacids, soy, high-fibre foods | Take calcium or iron at least 4 hours before or after levothyroxine (MedlinePlus) |
| Ciprofloxacin and similar quinolone antibiotics | Antacids and multivitamins with calcium, magnesium, aluminium, zinc or iron; dairy or calcium-fortified drinks on their own | Take those products at least 6 hours before or 2 hours after ciprofloxacin; don’t take it with milk or curd alone (MedlinePlus) |
| Doxycycline and other tetracyclines | Antacids, calcium, magnesium laxatives, iron | Antacids and calcium 1 to 2 hours apart; iron 3 hours before or 2 hours after (MedlinePlus) |
| Iron tablets | Calcium, zinc, antacids, tea, coffee, dairy | Calcium can reduce iron’s effect, and iron can reduce zinc’s (NHS). Take them at different times and ask your pharmacist about the gap. |
A simple routine for someone on a thyroid tablet and a calcium supplement: thyroid tablet on waking with water, breakfast 30 to 60 minutes later, calcium with lunch or dinner. Our hypothyroidism guide covers this in more detail, and if you take iron, see iron deficiency symptoms.
Vitamin K and warfarin
Warfarin works by blocking vitamin K, so a sudden change in vitamin K intake can make your INR swing. The NHS says leafy greens such as spinach and broccoli, which are high in vitamin K, can affect how warfarin works. You can still eat them; the key is to keep your intake fairly steady from week to week and talk to your warfarin clinic before making a big change.
The NHS also advises people on warfarin not to take vitamin K supplements or St John’s wort, to avoid cranberry juice and cranberry products, and not to drink large amounts of alcohol. Newer blood thinners (such as apixaban or rivaroxaban) don’t depend on vitamin K in the same way, but they have their own interactions, so check with your pharmacist.
Supplements that raise bleeding risk with blood thinners
If you take a blood thinner (warfarin, apixaban, rivaroxaban, dabigatran) or an antiplatelet (aspirin, clopidogrel), these supplements need a conversation with your doctor first:
- Ginkgo: the US National Center for Complementary and Integrative Health (NCCIH) warns it may increase bleeding risk in people on anticoagulants like warfarin.
- Garlic supplements: NCCIH says they may increase bleeding risk; tell your doctor, especially before surgery. Garlic in normal cooking isn’t the concern.
- Vitamin E: in a study of patients on warfarin, higher vitamin E levels in the blood predicted bleeding events. The study was small and observational, but it’s a reason to skip high-dose vitamin E capsules on a blood thinner.
- Fish oil (omega-3): the picture is more reassuring. A 2024 meta-analysis of 11 trials with more than 120,000 people found no overall rise in bleeding, though high-dose purified EPA did slightly increase it. Still mention it to your doctor.
Here’s the part many people miss: spacing these doses apart does not reduce the risk. The effect is on your blood’s ability to clot, which lasts through the day. Stop-and-start use is also risky with warfarin, because it makes your INR unpredictable. Tell your surgeon or dentist about all of these before any procedure.
St John’s wort: the herb that weakens other medicines
St John’s wort, sometimes taken for low mood, speeds up the breakdown of many medicines so they work less well. According to NCCIH, it can weaken:
- birth control pills (risk of unplanned pregnancy)
- warfarin (risk of clots)
- some antidepressants, HIV drugs, transplant drugs (ciclosporin), heart medicines (digoxin), seizure drugs, some cancer drugs and simvastatin
Combined with antidepressants or other medicines that affect serotonin, it can also cause serious serotonin-related side effects. If you’re being treated for depression, don’t add it on your own. If you’re struggling with your mood, Tele-MANAS (14416) is a free national helpline.
Vitamins with hidden risks: folic acid, vitamin A and biotin
Folic acid can hide vitamin B12 deficiency. The NHS notes that folic acid can improve anaemia symptoms so much that it masks a B12 deficiency, which can quietly damage nerves. That’s why doctors check B12 before starting folic acid treatment. (The standard 400 microgram folic acid supplement before and in early pregnancy is still recommended; the concern is about treating anaemia with folic acid without checking B12.)
Vitamin A has a ceiling. The NHS advises keeping total vitamin A from food and supplements at or below 1.5 mg a day, since years above that may weaken bones. In pregnancy, the NHS pregnancy guidance is clear: don’t take cod liver oil or supplements containing vitamin A (retinol), and avoid liver, because too much vitamin A can harm the baby’s development. Check the label of any multivitamin or “skin” supplement before you take it while pregnant or trying to conceive.
Biotin can throw off lab tests. High-dose biotin, sold for hair and nails, can interfere with some laboratory tests. The US FDA has received reports of falsely low troponin results, the blood test used to diagnose heart attacks, and the British Thyroid Foundation advises telling whoever takes your blood if you use supplements containing biotin before a thyroid test. Tell the doctor and lab if you take biotin, and ask whether to stop it before a blood test.
Herbs linked to liver injury or medicine clashes: kava and ashwagandha
Kava, used for anxiety, has been linked to rare but serious, sometimes fatal, liver injury, according to NCCIH. Don’t combine it with alcohol or sedatives such as sleeping pills, and avoid it if you have liver disease, drink regularly, or are pregnant or breastfeeding.
Ashwagandha is widely used in India. NCCIH says it may interact with medicines for diabetes, high blood pressure and thyroid conditions, with immunosuppressants and with sedatives. It isn’t recommended for people with thyroid disorders, should be avoided in pregnancy and while breastfeeding, and has been linked to rare cases of liver injury. If you’re on thyroid or diabetes medicine, check with your doctor before starting it, and watch your sugar readings closely if you do.
“Natural” doesn’t mean free of side effects. Our look at herbal detoxes explains why some popular herbal products carry real risks.
Your checklist before starting any supplement
- Make a list of everything you take: prescription medicines, over-the-counter tablets, vitamins, minerals, protein powders, Ayurvedic or herbal products, and home remedies like kadha you take daily.
- Show the list (or the actual bottles) to your doctor and pharmacist at every new prescription.
- Ask: “Does this need to be spaced from my other medicines? By how many hours?”
- Avoid taking several products with the same ingredient, such as a multivitamin plus a separate vitamin A or calcium tablet.
- Stop non-essential supplements before surgery only on your doctor’s advice, and tell the surgical team what you’ve been taking.
- Be extra careful if you’re pregnant, breastfeeding, over 65, on a blood thinner, or have liver or kidney disease.
When to see a doctor
Contact your doctor promptly if, after starting a supplement, you notice:
- unusual bruising, bleeding gums, nosebleeds that won’t stop, blood in urine, or black stools (especially on a blood thinner)
- yellowing of the eyes or skin, dark urine, pale stools or pain in the upper right abdomen (possible liver injury)
- shakiness, sweating, confusion or low sugar readings if you take diabetes medicine
- your thyroid, INR or other test results suddenly change for no clear reason
- feeling agitated, confused or suddenly unwell after adding a herb to an antidepressant
Call 112 for vomiting blood, a severe headache with confusion or weakness on one side, heavy bleeding that won’t stop, or chest pain.
Frequently asked questions
Can I take a multivitamin with my thyroid tablet?
Yes, but not at the same time if it contains calcium or iron. Leave at least 4 hours between them.
Is fish oil safe with aspirin or clopidogrel?
Large trials haven’t shown a clear rise in bleeding at usual doses, but high-dose EPA did slightly increase it. Mention it to your doctor so they can weigh it up.
Does taking my supplement at night avoid the interaction?
Only for absorption interactions (minerals with thyroid tablets or antibiotics). It doesn’t help with bleeding risk or with herbs like St John’s wort that change how your liver handles drugs.
Are Ayurvedic and herbal products exempt from interactions?
No. Herbs contain active compounds that can interact with medicines. Bring them to your doctor or pharmacist like any other medicine.
Should I stop biotin before a blood test?
Tell your doctor and the lab you take it and ask what they advise. Some tests are affected and some aren’t.
This article is for general information and is not a substitute for advice from a doctor. If you have symptoms or a health condition, please consult a qualified healthcare professional.
