There are HIPPA laws protecting your privacy in medical matters. I don't know what the MEPS questionnaire specifically asked, but you did not take any drugs illegally. If the questions were generic in nature, you are probably Okay since it was a short term occurrence and not a life changing event. If they specifically asked if you have ever been treated for depression or taken Lexapro, and you answered "no", you may have a problem. As GLW stated, call your recruiter and mention that you have thought of something that you didn't include in your MEPS. If it is important, he'll know how to handle it. It appears to be just a case of forgetting something that was not of significance in your life, which is understandable. It was not a crime, an intent to defraud, or anyting else nefarious. Good Luck.