When I was in the Infantry (Active Duty) I was also the urinalysis NCO for a couple years....they wont test you unless they have resonable suspicion that your on...they being your commander....Not sure about the "marines" but it costs the army around $1000.00 dollars to test "one" soldier. Every other piss test you take is for recreational drugs. We just took a piss test in the reserves a few months ago and a couple of my buddies were on....they all passed. If your heading to MEPS and your still worried about it....go to a doctor and talk to him...ask to be tested and have bloodwork done....until you can pass..then go in....military is always going to be their.
How long ago was your last cycle?....