Quick answer
Ask your pharmacist or doctor before combining. The prescription niacin label reports small drops in platelet count and a longer clotting time, and advises caution and close monitoring for people on anticoagulants. NIH notes that nicotinamide at 500 mg a day or more can cause easy bruising and more bleeding from wounds.1,2
Key facts
- At 2,000 mg a day, prescription niacin lowered platelet counts by about 11% on average.1
- It lengthened prothrombin time, a clotting test, by about 4% on average.1
- The prescription niacin leaflet asks people to tell their doctor if they take blood thinner medicines.1
- MedlinePlus lists unusual bleeding or bruising among the side effects of niacin to report right away.3
Ask your pharmacist or doctor first
The prescription niacin label reports small, dose-related drops in platelet count and rises in prothrombin time, a clotting test, and advises caution and close monitoring of both in people taking anticoagulants. NIH notes that nicotinamide at 500 mg a day or more can cause easy bruising and more bleeding from wounds.1,2
On the medicine's label
What is documented
The label for extended-release nicotinic acid describes two small, dose-related changes seen in trials: fewer platelets and a longer prothrombin time, a clotting test. It advises caution when the drug is used with anticoagulants, with close monitoring of both measures.1
The same label lists arterial bleeding and an active stomach ulcer among the reasons not to take prescription niacin, and says people having surgery should be evaluated carefully.1
MedlinePlus adds that anyone taking niacin should tell the doctor or dentist before any surgery, including dental surgery.3
Nicotinamide and bleeding
Nicotinamide, the form that does not flush, has its own effect at high doses. NIH says 500 mg a day or more can cause easy bruising and increase bleeding from wounds, and in small studies of people on dialysis, 500 to 1,500 mg a day lowered platelet counts.2,4
Which blood thinners
The niacin label speaks of anticoagulants as a group and does not name individual drugs. The sources on this page do not address antiplatelet medicines separately. That is not a finding either way, so ask your pharmacist about your own medicine.1
Supplement amounts
The documented effects come from prescription doses of up to 2,000 mg a day, and from nicotinamide at 500 mg a day or more. The upper limit for niacin from supplements is 35 mg a day for adults. The sources do not say what smaller amounts do alongside a blood thinner.1,2
What to ask your pharmacist
- Does any product I take contain niacin, nicotinic acid or nicotinamide, and how much?
- Do I need extra blood tests if I take both?
- Should I tell my surgeon or dentist about niacin before a procedure?
Questions people ask
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Sources
- Niacin extended-release tablets: prescribing information and patient information (Amneal Pharmaceuticals). DailyMed, U.S. National Library of Medicine. Published September 18, 2026. Opened 2026-10-07.
- National Institutes of Health, Office of Dietary Supplements. Niacin: Fact Sheet for Consumers. Updated March 22, 2021. Opened 2026-10-07.
- MedlinePlus, U.S. National Library of Medicine. Niacin: drug information (American Society of Health-System Pharmacists). Last revised April 15, 2026. Opened 2026-10-07.
- National Institutes of Health, Office of Dietary Supplements. Niacin: Fact Sheet for Health Professionals. Updated November 18, 2022. Opened 2026-10-07.
This page is information, not medical advice. Talk to your doctor or pharmacist about your own situation.