The U.S. Anti-Doping Agency said Solo will still be able to compete in this month's 2012 Olympic Games in London despite the infraction.
The agency said the 30-year-old goalkeeper tested positive for the prohibited performance-enhancing substance Canrenone during an out-of-competition test conducted June 15.
Solo said the result was due to prescription medication she was taking.
"I took a medication prescribed by my personal doctor for pre-menstrual purposes that I did not know contained a diuretic," she said. "Once informed of this fact, I immediately cooperated with USADA and shared with them everything they needed to properly conclude that I made an honest mistake, and that the medication did not enhance my performance in any way."
Canrenone is classified as a "specified" substance under international rules, meaning its use can carry a reduced sanction.