AbstractSTUDY QUESTIONWhat is the evidence for over-the-counter antioxidant supplements for male infertility?SUMMARY ANSWERLess than half of over-the-counter antioxidant supplements for male fertility patients have been tested in a clinical trial, and the available clinical trials are generally of poor quality.WHAT IS KNOWN ALREADYThe prevalence of male infertility is rising and, with this, the market for supplements claiming to improve male fertility is expanding. Up to now, there is limited data on the evidence for these over-the-counter supplements.STUDY DESIGN, SIZE, DURATIONAmazon, Google Shopping and other relevant shopping websites were searched on 24 June 2022 with the following terms: ‘supplements’, ‘antioxidants’, ‘vitamins’, AND ‘male fertility’, ‘male infertility’, ‘male subfertility’, ‘fertility men’, ‘fertility man’. All supplements with a description of ingredients in English, Dutch, French, Spanish, or German were included. Subsequently, Pubmed and Google Scholar were searched for studies that included the supplements.PARTICIPANTS/MATERIALS, SETTING, METHODSInclusion criteria were supplements with antioxidant properties, of which the main purpose was to improve male fertility. Included supplements must be available without a doctor’s prescription. Supplements containing plant extracts were excluded, as well as supplements of which the content or dosage was not clear. The ingredients, dosage, price and health claims of the supplements were recorded. We assessed whether substances in the supplements exceeded the recommended dietary allowance (RDA) or tolerable upper intake level (UL). All clinical trials and animal studies investigating included supplements were selected for this review. Clinical trials were assessed for risk of bias with a risk of bias tool appropriate for the study design.MAIN RESULTS AND THE ROLE OF CHANCEThere were 34 eligible antioxidant supplements found, containing 48 different active substances. The average price per 30 days was 53.10 US dollars. Most of the supplements (27/34, 79%) contained substances in a dosage exceeding the recommended daily allowance (RDA). All manufacturers of the supplements made health claims related to the improvement of sperm quality or male fertility. For 13 of the 34 supplements (38%), published clinical trials were available, and for one supplement, only an animal study was found. The overall quality of the included studies was poor. Only two supplements were tested in a good quality clinical trial.LIMITATIONS, REASONS FOR CAUTIONAs a consequence of searching shopping websites, a comprehensive search strategy could not be formulated. Most supplements were excluded because they contained plant extracts or because supplement information was not available (in an appropriate language).WIDER IMPLICATIONS OF THE FINDINGSThis is the first review that gives an insight into the market of male fertility supplements as available to infertility patients and other men seeking to improve their fertility. Earlier reviews have focused only on supplements with published clinical trials. However, we show that more than half of the supplements have not been tested in a clinical trial. To our knowledge, this review is the first to assess the dosage of supplements in relation to the RDA. In agreement with the literature, we found that the evidence on male fertility supplements is generally of poor quality. This review should urge pharmaceutical companies to evaluate their products in randomized controlled trials in order to provide people with substantiated information.STUDY FUNDING/COMPETING INTEREST(S)The research position of W.R.d.L. is funded by an unrestricted grant from Goodlife Pharma. W.R.d.L., K.F., and J.P.d.B. are in the research team of a clinical trial on Impryl®, one of the supplements included in this review.REGISTRATION NUMBERN/A.