Supplements and medication — What you need to know

Important: this page is informational and does not replace advice from a doctor or pharmacist. If you use prescription medication, always discuss supplement use with your pharmacist.

How supplements and medication can influence each other

There are three main ways supplements and medication can interact:

• Absorption — certain minerals bind to medication in the gastrointestinal tract, reducing how much of the medication is absorbed. This is a timing issue: the substances are taken together when they should be taken apart.
• Effect — some supplements strengthen or weaken the pharmacological effect of a medication. This interaction occurs regardless of timing.
• Breakdown — some plant‑based substances affect liver enzymes responsible for breaking down medication, causing blood levels of the medication to become higher or lower than intended.

Blood thinners

(Coumarins such as acenocoumarol and phenprocoumon)

Coumarin blood thinners work by inhibiting Vitamin K, which plays a role in blood clotting. More Vitamin K means the blood thinner becomes less effective. Vitamin K is found mainly in green leafy vegetables such as kale, spinach, Brussels sprouts, broccoli and lettuce. Fluctuations in your intake of these vegetables can therefore influence how well the blood thinner works.

• Vitamin K — supplements containing Vitamin K can directly affect the action of Coumarins. This also applies to Vitamin K from food. People using Coumarins are monitored with INR tests. If your Vitamin K intake changes, always discuss this with your care provider.
• Vitamin E — high doses (above 400 IU per day) can influence blood clotting and strengthen the effect of blood thinners. At normal supplement doses, this risk is limited.
• Omega‑3 (EPA and DHA) — has a mild blood‑thinning effect at normal supplement doses. If you use blood thinners, always discuss Omega‑3 with your doctor or pharmacist.
• Botanicals — St. John’s Wort, Ginkgo Biloba, Ginseng, Garlic and Ginger can affect how blood thinners work. Using these herbs alongside blood thinners always requires medical guidance.
• Newer blood thinners (DOACs) — rivaroxaban, apixaban and dabigatran work differently from Coumarins. Vitamin K plays no role with DOACs, but the same rule applies: always discuss supplement use with your pharmacist.

Antibiotics

• Mineral supplements (Magnesium, Calcium, Iron, Zinc) — these minerals can form insoluble complexes with certain antibiotics in the gastrointestinal tract, significantly reducing antibiotic absorption. This applies especially to tetracyclines (doxycycline, minocycline) and fluoroquinolones (ciprofloxacin, levofloxacin).

Practical advice: take mineral supplements at least 2 hours before or 4–6 hours after the antibiotic. Always tell your pharmacist you use mineral supplements when collecting antibiotics.

Thyroid medication (levothyroxine)

• Iron, Calcium and Magnesium — bind to levothyroxine in the gastrointestinal tract, reducing absorption of the thyroid hormone. Always take these supplements and levothyroxine at least 2–4 hours apart. This also applies to calcium from dairy products.

Timing is especially critical with thyroid medication. Always consult your pharmacist about optimal timing.

Statins (cholesterol‑lowering drugs)

• Vitamin D — research links low Vitamin D status to muscle complaints in statin users. Whether supplementation is relevant for you is something to discuss with your doctor.
• CoQ10 — statins inhibit an enzyme involved in producing CoQ10. There is scientific debate about whether CoQ10 supplementation is useful for statin users — consult your doctor or pharmacist.
• Red yeast rice — contains Monacolins that are structurally identical to the statin lovastatin. Combining red yeast rice with a statin can lead to an excessively high effective dose. Never use red yeast rice alongside statins without medical advice.

Antacids (proton pump inhibitors such as omeprazole)

Long‑term PPI use reduces stomach acid production, which affects the absorption of certain nutrients.

• Vitamin B12 — stomach acid is needed to release Vitamin B12 from food proteins. Long‑term PPI use can reduce B12 absorption. With long‑term use, monitoring via blood tests is advisable — discuss this with your GP.
• Magnesium — long‑term PPI use is associated with lower blood magnesium levels. If you have symptoms that may indicate low magnesium, discuss this with your GP.
• Iron and Calcium — absorption of non‑heme iron and calcium partly depends on stomach acid. Long‑term PPI use can reduce absorption of both.

Metformin (for type 2 diabetes)

• Vitamin B12 — metformin reduces B12 absorption in the gut. With long‑term use, monitoring B12 status is recommended. Discuss this with your GP or diabetes nurse.

Diuretics (water pills)

Diuretics influence fluid and mineral excretion through urine. Depending on the type, this can affect levels of potassium, sodium and magnesium. If you use diuretics and are considering a mineral supplement, always discuss this with your doctor or pharmacist — needs and safety vary per medication.

Painkillers

• Ibuprofen and other NSAIDs — ibuprofen has a mild blood‑thinning effect. Combining it with Omega‑3 or other supplements with blood‑thinning properties (Vitamin E, Garlic) can theoretically increase bleeding risk. With short‑term, occasional use the risk is limited; with regular NSAID use, consulting a pharmacist is advisable.
• Paracetamol — has relatively few interactions with common supplements at normal doses. High doses of Vitamin C (above 1 gram per day) can affect paracetamol breakdown. With normal use, most supplement combinations are not problematic. If in doubt, ask your pharmacist.

Antidepressants (SSRIs)

SSRIs increase serotonin availability in the brain. Certain supplements also influence serotonin metabolism. Together, this can lead to excessive serotonin activity (serotonin syndrome).

• 5‑HTP — a direct precursor to serotonin. Never combine 5‑HTP with SSRIs without medical advice.
• Tryptophan — an amino acid the body converts into serotonin. Combining it with SSRIs requires medical guidance.
• SAMe — affects serotonin metabolism. Combining it with SSRIs requires medical guidance.
• St. John’s Wort — increases serotonin activity and affects SSRI breakdown via liver enzymes. This combination is contraindicated — never use St. John’s Wort with SSRIs without medical advice.

Sleep medication and sleep‑related drugs

• Melatonin — has its own sleep‑promoting effect. Combining it with prescribed sleep or sedative medication (benzodiazepines, z‑drugs such as zolpidem) can strengthen sedation. Always discuss melatonin with your doctor or pharmacist if you use sleep medication.
• Magnesium — commonly used for relaxation and sleep. When combined with muscle relaxants or sedative medication, consulting a pharmacist is advisable.
• Valerian root, Passionflower, Hops — botanicals used in sleep formulas can strengthen the sedative effect of sleep or anxiety medication. Always discuss use with your doctor or pharmacist.

Multivitamins and medication

Multivitamins often contain small amounts of minerals such as calcium, magnesium, iron and zinc. These can cause the same interactions as standalone mineral supplements — especially with antibiotics and thyroid medication. Always consider the mineral content of a multivitamin when planning timing around medication.

Multivitamins containing Vitamin K are particularly relevant for people using Coumarin blood thinners. Check the label for Vitamin K content.

St. John’s Wort

St. John’s Wort (Hypericum perforatum) deserves special mention. It is one of the most potent herbs in terms of medication interactions. It activates liver enzymes that break down many medications, lowering their blood levels. This applies to the contraceptive pill, antidepressants (SSRIs), blood thinners, HIV medication, immunosuppressants and anti‑epileptic drugs.

Never use St. John’s Wort alongside prescription medication without medical advice.

When to always consult your pharmacist

Always contact your pharmacist before starting a supplement if you:

• Use blood thinners (Coumarins or DOACs)
• Use thyroid medication (levothyroxine)
• Use antidepressants (SSRIs or MAO inhibitors)
• Use immunosuppressants
• Use anti‑epileptic drugs
• Use 3 or more prescription medications
• Are pregnant or breastfeeding and use medication
• Want to stop or adjust a supplement while using medication

Your pharmacist knows your medication profile and has access to up‑to‑date interaction databases. This is always the best first step.

Frequently asked questions

Can I use Magnesium with blood thinners?

Magnesium has no direct interaction with Coumarins at normal doses. Still, always discuss supplement use with your pharmacist.

Can I use Vitamin D with statins?

There is no direct contraindication. Research links low Vitamin D status to muscle complaints in statin users. Discuss relevance with your doctor.

Can I use St. John’s Wort if I take medication?

St. John’s Wort interacts strongly with many medications. Never use it without medical advice.

How far apart should I take Iron and thyroid medication?

At least 2–4 hours. Preferably take levothyroxine in the morning on an empty stomach, and Iron later in the day.

Can I use Omega‑3 with blood thinners?

Omega‑3 has a mild blood‑thinning effect. Always consult your doctor or pharmacist.

Can long‑term PPI use lead to Vitamin B12 deficiency?

Yes. Long‑term PPI use reduces B12 absorption. Monitoring via blood tests is advisable.

Can I use 5‑HTP with antidepressants?

No — not without medical advice. 5‑HTP affects serotonin metabolism and should not be combined with SSRIs.

My multivitamin contains Vitamin K — is that a problem with blood thinners?

Vitamin K can affect Coumarin blood thinners. Check the label and discuss with your pharmacist.

Can sleep medication and Melatonin be combined?

Melatonin can strengthen sedation. Always discuss with your doctor or pharmacist.

Where can I get personal advice?

Your pharmacist is the designated specialist. At Gezondheid aan huis, our certified pharmacists are also available by phone, WhatsApp or email.