Testosil contains KSM-66 ashwagandha (300 mg per serving), D-aspartic acid, zinc and vitamin D. We review the evidence, the doses, cautions and the guarantee terms.
Testosil is our top pick among testosterone-support formulas, but it has never been tested as a finished product. Each 4-capsule serving has 300 mg of KSM-66 ashwagandha, the ingredient with the best (still small) evidence, plus 2,000 mg D-aspartic acid, zinc, vitamin D and other ingredients, all with disclosed doses. Several doses fall short of the research. It's sold with what the company calls a lifetime money-back guarantee, which refunds unopened bottles plus one opened bottle.
There's no trial of Testosil itself. Its main support is KSM-66 ashwagandha: in a 2015 trial, 57 men aged 18 to 50 doing resistance training took 300 mg twice daily (600 mg a day, equal to two Testosil servings) for 8 weeks. Testosterone rose by about 96 ng/dL versus 18 ng/dL on placebo, but the trial was small, the extract was supplied by its maker, and end-of-study levels weren't significantly different between the groups.
The other ingredients are weaker. D-aspartic acid at 3 g a day didn't change testosterone in trained men, and Testosil's 50 mg of fenugreek is a twelfth of the 600 mg used in the best-known (maker-funded) fenugreek trial. Zinc and vitamin D mainly help men who are deficient. For more on the main herb, see does ashwagandha work.
Amounts are from the label figures on a 2023 lab certificate linked from the official site; the current label could differ. One serving already reaches the adult upper limits for zinc and vitamin D, so two servings, or one plus a multivitamin, goes over them.
Testosil is advertised with a lifetime money-back guarantee. The terms (checked October 2026) refund unopened bottles plus one opened bottle, exclude shipping and are limited to one order per customer. Some wording on the guarantee page is inconsistent, so confirm with customer service before returning.
Ashwagandha has been linked to rare liver injury and can raise thyroid hormone levels. It and several other ingredients can interact with diabetes, blood pressure, sedative, immunosuppressant and blood-thinning drugs. See Testosil side effects for details, and if you have symptoms of low testosterone, get tested; our guide to low testosterone symptoms explains when to see a doctor.