where i live, fentanyl requires a more sensitive/expensive test to detect than other opiates like OC, morphine, heroin, etc. which means that a lot of addicts and ppl on MMT, or who know that they will be tested regularly for other reasons, will ONLY do fentanyl.
not 100% sure that this is the truth, but it's what i've heard from several sources. so you may be in the clear...