I agree with oldnavy,
"Honesty is the best policy"!!

I will tell you that I struggled with this exact dilemma when I first went to see my PM DR.
I was previously going to a methadone clinic before seeing him. I did not want ot reveal this to him b/c I was afraid of what he might think of me. They made me sign an agreement with them, and doing so meant that I must be completly honest with the Dr at all times.
I decided to tell him, afterall it is illegal to lie to your Dr to get narcotics, so I told him everything, the reason I was going to meth clinic was b/c, ny GP, told me that I had no signs of injury to my back, this is after he kept me on vicodin for 2 years before kicking me to the curb.
I also told him that I debated whether or not to be honest with him about this, due to all the stigma associated with the clinics.
He really appreciated my being candid with him, and told me that he would had found out, since at the end of our session, he was going to submit me to urine test. Then when they found the methadone, I would had been dismissed from his care.
He also said that my previous dose of methadone was 100mg a day, and he had to give me something that would be eqaul to it, so had I lied to him, the normal dose he starts his pts out on, is 20mg morphine per day. That would had set me off in w/d,s, he did end up giving me 200mg morphine per day, and at the end of our session, he again shook my hand and thanked me for being honest!

I know that your situation is somewhat different than mine, but, I still believe that being honest always pays off in the long-run!!

I wish you all the best!!
Terri