Having gone through this process before, I can offer a little insight. In addition to things like your criminal record, they want to investigate anything that could be used against you to manipulate you into possibly doing questionable activity.
The issue about any single item of interest in the investigation is could that thing be used to manipulate you?
For example, if you have a history of theft but never get caught- could you be blackmailed into providing classified information in exchange for keeping that history of yours private from your boss? Also there are other mundane things, such as whether you are heavily in debt- people in debt tend to do illegal things for extra money that they otherwise wouldn't do- like give classified documents in exchange for cash. Or perhaps they break the law, or cheat on their spouse- and so they are blackmailed into providing classified information in exchange for keeping things quiet.
Your medical condition could be used against you if you feel it's embarrassing or something you would rather not reveal to your employerinvestigators. Being up front by telling them this information in advance removes the possibility that you can be blackmailed about it or embarrassed to have the condition exposed. Your honesty and integrity is important with respect to the security clearance, and your medical condition is something you should be willing to provide if they ask questions.
Do not let them find out after the fact that you are taking medications. Also, see your doctor and get the meds. By attempting to circumvent the issue- you make it clear you are either embarassed or can be manipulated into doing things to keep the issue private from your employer.