It is definitely a felony! I had a friend years ago who did almost the same thing only she added a refill to a legit script, back in the days where the Dr would leave the refill line blank if they weren't giving you any refills. She went to the pharmacy, got the first script, then when she went to refill it, the pharmacist for some reason said that he needed to call the Dr to just double check on the refill (it was a schedule 3 drug, I believe) Anyway, of course the Dr's office said that they had not authorized any refills and she was immediately picked up by 2 US Marshalls. She spent a day in jail until she could get bailed out and got 3 years probation. She was told she could have spent 12 years in jail. This all happened years ago before they started coming down hard on drug abuse too.
I wish her luck!