Major League Baseball in a memo warned about the "very real risk" of over-the-counter sexual-enhancement pills after at least two players this year were suspended for performance-enhancing drugs and said the banned substances found in their urine came from the unregulated products, sources familiar with the situation told ESPN.

The use of over-the-counter pills, which are often sold at gas stations, is prevalent among baseball players, according to multiple sources. It prompted the league to send out a memo on Monday that outlines the risk of consuming non-NSF-certified supplements.

The memo, obtained by ESPN, warns that "these products are often contaminated with prohibited and unsafe ingredients" and that players are subject to discipline even if they inadvertently ingested a banned substance.

"Sexual or male enhancement products present a very real risk for drug-tested players," the memo said, "and the high likelihood for contamination or unidentified ingredients in these products underscores the importance of consuming only those products that are NSF Certified for Sport."

In the memo, which was sent to major and minor league players and redistributed by the MLB Players Association to ensure its members received it, MLB suggested that players who "suffer from erectile dysfunction or other legitimate issues related to sexual performance ... speak to a licensed physician about the various prescription medications (e.g., Viagra, Cialis, Levitra) available to treat those conditions."

Over-the-counter sexual-enhancement pills are part of the unregulated supplement industry, which is estimated to be worth more than $30 billion a year. The Food and Drug Administration in July added 10 products to a list of more than 250 tainted sexual-enhancement supplements. Former NBA star Lamar Odom fell into a coma after a significant dose of so-called "herbal Viagra," a sexual-enhancement pill.

"We know from experience," the league memo said, "that a number of these sexual or male enhancement products -- which are sold online, at retail stores, and on the black market, both in the United States and internationally -- contain anabolic steroids and other prohibited substances.

"For this reason," the memo continued, "we strongly urge players against taking any sexual or male enhancement product, from any source."