However, sildenafil and chemicals similar to sildenafil have been found as adulterants in many supplements which are sold as herbal viagra or "natural" sexual enhancement products.[1][6][7] The United States Food and Drug Administration has warned consumers that any sexual enhancement product that claims to work as well as prescription products is likely to contain such a contaminant.[7] Scientists estimated that >60% of the consumed sildenafil in the Netherlands is from illegal sources such as adulterated dietary supplements.[8]