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Doctor's Best vs Source Naturals Nattokinase (2026)
Disclosure: We earn commissions on purchases made through our links. This never influences our scores. Editorial policy
The Verdict
Doctor's Best takes it on our rubric, 90 to 87 on our 0-100 execution score, and holds our Top Pick badge in the nattokinase scorecard. The gap is our value pillar, 24 against 20: at about $0.18 per day against $0.33, it is close to half the cost for the same branded ingredient at the same activity level. Source Naturals wins transparency by a point (23 vs 22) and, per our notes, was one of the earliest US brands to license NSK-SD; its capsules are vegan. In our view, when two products use the identical branded raw material at the identical dose and neither holds an independent certification, cost per day is the rational tiebreaker, which makes Doctor's Best the default.
Take Source Naturals if you specifically want the vegan capsule or prefer its California manufacturing. Both are soy-derived, so neither suits a soy allergy, and nattokinase interacts with blood-thinning medication, which makes this a category to clear with your doctor first.
Nattokinase 2,000 FU
Doctor's Best
NSK-SD Nattokinase 100mg
Source Naturals
Head-to-Head Comparison
| Category | Nattokinase 2,000 FU Doctor's Best | NSK-SD Nattokinase 100mg Source Naturals |
|---|---|---|
| Brand Score | 90/100Winner | 87/100 |
| Dosing & Form | 25/25Winner | 25/25 |
| Purity | 19/25Winner | 19/25 |
| Value | 24/25Winner | 20/25 |
| Transparency | 22/25 | 23/25Winner |
| Cost/Day | $0.18Winner | $0.33 |
| Dose/Serving | 100mg | 100mg |
| Form | NSK-SD Nattokinase capsule (2,000 FU) | NSK-SD Nattokinase vegan capsule (2,000 FU) |
| Third-Party Tested | No | No |
| Proprietary Blend | No | No |
Why This Comparison Matters
Both of these products use NSK-SD, the branded nattokinase from Japan Bio Science Laboratory that our data identifies as the exact ingredient used in the published trials, at the same 100mg and 2,000 FU per capsule. Both name the source on the label and disclose FU activity, which is the detail that separates a serious nattokinase product from a vague one. Both earn a full 25 on our dosing and form pillar and an identical 19 on purity verification.
So this is a bottle-and-price comparison. Doctor's Best ships 90 capsules for $15.99, about $0.18 per day. Source Naturals ships 60 capsules for $19.95, about $0.33 per day.
Neither carries USP or NSF certification. Both are manufactured in GMP facilities, Source Naturals in Scotts Valley, California with in-house testing.
Detailed Score Breakdown
Nattokinase 2,000 FU
Doctor's Best
100mg / 2,000 FU matches the dose used in Kim 2008 and Jensen 2016 RCTs
Doctor's Best uses NSK-SD branded nattokinase from Japan Bio Science Laboratory, manufactured in a GMP facility
$0.18/day at 1 capsule (90 caps for ~$16) after a price drop - excellent value for a clinically dosed NSK-SD product
FU activity disclosed, NSK-SD source identified, vitamin K2 removed status confirmed
Doctor's Best is one of the more transparent NSK-SD products on the market at a reasonable price
NSK-SD Nattokinase 100mg
Source Naturals
Genuine NSK-SD at 100mg / 2,000 FU - the exact ingredient used in the published clinical trials
Source Naturals manufactures in a GMP-certified facility in Scotts Valley, CA with in-house testing
$0.33/day at 1 capsule (60 caps for ~$20) - moderate pricing for a confirmed NSK-SD product
NSK-SD source explicitly named, FU activity disclosed, full ingredient breakdown
Source Naturals was one of the earliest US brands to license NSK-SD and remains a benchmark for label transparency
How We Compared These Products
Every product in our database is scored on four equally-weighted pillars: dosing accuracy and form quality, purity verification (third-party testing), cost per clinically effective dose (not cost per pill), and label transparency. Each pillar is worth 25 points for a total of 100.
Cost per effective dose is calculated using the clinically studied dose from published research, not the manufacturer's suggested serving. If a product requires multiple servings to reach the dose used in clinical trials, that cost is reflected in the value score.
For a full explanation of our scoring methodology, see our methodology page. Prices were last checked on the dates listed for each product and may have changed.
Frequently Asked Questions
Do both products use the same nattokinase ingredient?
Yes. Both use NSK-SD branded nattokinase from Japan Bio Science Laboratory at 100mg and 2,000 FU per capsule, and both name the source and disclose FU activity on the label. Per our data, that is the exact ingredient and dose used in the published clinical trials.
What does FU mean on a nattokinase label?
FU stands for fibrinolytic units, the measure of enzymatic activity. It matters more than the milligram weight, because two products at 100mg can carry very different activity. Both products here disclose 2,000 FU, which our notes match to the dose used in the trials they cite.
Which nattokinase is cheaper per day?
Doctor's Best, at about $0.18 per day against Source Naturals at about $0.33. Doctor's Best ships 90 capsules for $15.99; Source Naturals ships 60 for $19.95. Both are one capsule per day at the studied dose.
More Nattokinase Comparisons
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FDA Disclaimer: These statements have not been evaluated by the Food and Drug Administration. Dietary supplements are not intended to diagnose, treat, cure, or prevent any disease. Always consult with a qualified healthcare professional before starting any supplement regimen.