Major League Baseball has warned players in a memo about the "very real risk" of over-the-counter sexual-enhancement pills resulting in positive tests for performance-enhancing drugs.

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, according to sources.

The use of over-the-counter pills, which are often sold at gas stations, is prevalent among baseball players.

The memo 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."