Testosil's formula pairs KSM-66 ashwagandha (300 mg per serving) with D-aspartic acid, zinc and vitamin D. Here are the research, the doses, the safety cautions and the guarantee terms.
The finished Testosil product has never been tested, and yet it's our top pick among testosterone-support formulas. The standout is KSM-66 ashwagandha at 300 mg per 4-capsule serving, which has the best evidence of anything inside, though that evidence is still small. The rest of the serving includes 2,000 mg D-aspartic acid, zinc, vitamin D and more, every dose disclosed. Several are lower than research used. The company's lifetime money-back guarantee covers unopened bottles and one opened bottle.
No one has run a trial on Testosil. What supports it is KSM-66 ashwagandha research from 2015: 57 men, 18 to 50 years old and doing resistance training, took 300 mg twice a day for 8 weeks. That totals 600 mg daily, or two Testosil servings. Their testosterone went up about 96 ng/dL while the placebo group's went up 18 ng/dL. Keep in mind the small size, the extract being supplied by its maker, and the lack of any significant difference in end-of-study levels between groups.
Support for the remaining ingredients is thinner. D-aspartic acid at 3 g a day left testosterone unchanged in trained men. The best-known fenugreek trial, which the maker funded, used 600 mg, and Testosil has only 50 mg, a twelfth of that. Zinc and vitamin D are mostly useful to men who are deficient. Our piece on whether ashwagandha works covers the main herb.
The figures are taken from label amounts on a 2023 lab certificate that the official site links to. Today's label could be different. One serving by itself reaches the adult upper limits for zinc and vitamin D, and two servings, or one plus a multivitamin, exceed them.
The company advertises a lifetime money-back guarantee. As of October 2026, the terms refund unopened bottles plus one opened bottle, leave out shipping, and allow one order per customer. The guarantee page words some things inconsistently, so confirm with customer service before you send anything back.
Rare liver injury has been reported with ashwagandha, and it may push thyroid hormone up. It, along with several other ingredients, can interact with drugs for diabetes and blood pressure, sedatives, immunosuppressants and blood thinners. We go into detail in Testosil side effects. Symptoms of low testosterone call for a blood test, and our guide to low testosterone symptoms explains when a doctor visit makes sense.