Nattokinase
NSK-SD, Natto enzyme
Forms: Enzyme from natto (fermented soybeans) sold in capsules; activity is labelled in fibrinolytic units (FU).
Enzyme from natto, a Japanese fermented soybean food, sold as a 'natural blood thinner'. It increased clot breakdown in a small human study; a long-term trial found no effect on arteries, blood pressure or lab tests.
Legal status: Sold freely as a supplement in most countries; varies by country. Not on the WADA Prohibited List.
What it gives
What people use it for and what the sources report. Strength is a general category from the literature: mild, clear or strong.
Purpose
Popular among steroid users who take it to 'thin the blood' when hematocrit is high, and among people seeking heart health. There is no evidence that it lowers hematocrit or clot risk; it adds bleeding risk, especially with aspirin.
What people seek and what studies show
A single intake in 12 healthy young men raised fibrin breakdown products and prolonged a clotting time for a few hours, all within normal ranges. In NAPS (265 people, median age 65) artery wall thickening and stiffness, blood pressure, blood flow (rheology), clotting and other lab markers did not differ from placebo. No studies in athletes or steroid users.
Effects
No clear training effect in the sources.
Evidence: Limited human data — some human studies, small or short
Possible side effects
Possible does not mean certain: whether they appear depends on dose, duration, individual response and monitoring. Below — what to watch for and how to notice it early.
2 possible side effects
level · how often it is reportedWhat the levels mean
Level: general category from the literature, not your personal risk
How often
- common
- reported often
- possible
- reported in some people
- rare
- reported occasionally
How to lower the risks
For 1 of 2 possible side effects there are specific ways to lower the chance: everyday measures, supplements and options a doctor may consider. For the rest — general advice. No measure removes a risk completely.
Do not combine blood-thinning products (aspirin, painkillers such as ibuprofen, high intakes of fish oil, nattokinase, vitamin E) without a doctor; warn doctors and dentists before procedures; alcohol adds to stomach-bleeding risk. On anticoagulants, contact sports are usually put on hold.
How each option helps (4)
- Working with a doctor — Checks which blood-thinning products are combined (aspirin, ibuprofen-type painkillers, fish oil, vitamin E) and whether stomach protection is needed; tell doctors and dentists before procedures.
- Avoiding alcohol and other liver stressors — Alcohol adds to stomach-bleeding risk with aspirin and painkillers such as ibuprofen; cutting it out lowers that part of the risk.
- Omeprazole and other proton-pump inhibitors (PPIs) — Lowered aspirin stomach ulcers, and upper-gut bleeding on aspirin plus clopidogrel, in trials; not bruising or brain bleeds. Who needs it is a doctor's call (age, past ulcer).
- Famotidine (H2 blockers) — Cut new ulcers in low-dose aspirin users in a trial, but bleeding itself was not the outcome studied; guidelines name PPIs for bleeding cover. A doctor decides.
Low-level side effects
Check-ups and blood tests
Which tests and check-ups to do and when. This is a guide — agree the exact schedule with a sports physician.
Which doctors can help
Get medical help at once if
- Black or bloody stools, vomiting blood or “coffee grounds”, bleeding that won’t stop, a sudden severe headache, any head blow on blood thinners (not aspirin alone) — urgent care or your local emergency number.
In an emergency, call your local emergency number.
Myth & fact
“Nattokinase thins the blood, so it fixes the thick blood (high hematocrit) that steroids cause.”
It acts on fibrin, not on red cells, so hematocrit stays high; a long-term trial showed no change in clotting or blood-flow tests. With aspirin it adds bleeding risk; a high hematocrit needs a doctor.
Combinations
Nattokinase breaks down fibrin, while aspirin and NSAID painkillers make platelets less sticky. A bleed in the brain was reported in a woman who added nattokinase to aspirin. Unusual bruising, black stools or a sudden severe headache need urgent care.
Aspirin (acetylsalicylic acid)NSAID tablets (ibuprofen, naproxen, diclofenac and others)Sources
Possible side effects by body system, combinations, how to lower the risks and which tests to do — free.
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