As long as the reagent touches just a few mcgs of fentanyl it will react accordingly. Now with something like cocaine, MDMA, or Crystal Meth, you are more likey to run into a nbome than fentanyl. Unless the dealer wants to kill you, you will never see fentanyl in stimulants. I mean if your dealer has a supply of fentanyl, and stores it near their other drugs its possible it may contaminate all his other drugs. Just to be safe I would personally test all my drugs if I had the right resources, despite how unlikely fentanyl contamination may be in meth or acid.
Also I would say when testing a drug with a reagent to test a piece from the outside, and inside of the sample. This is because if fentanyl was accidentally mixed into the wrong drug, only small spots would be present. Because of this there's a chance you will test a fentanyl free portion of your sample and will consume the product without knowing it has fentanyl. Testing several different parts of a sample can reduce the chance of you missing a "hotspot" of fentanyl
There are plenty of sites (I wont name here) which will provide free/cheap naloxone kits for nasal, and IM use, which although your not using heroin or a drug at risk or fentanyl contamination, could save your life if the unlikely happens.
Keep in mind that their exist many other better methods of testing drugs for contamination other than reagents, so if you really want to make sure, think about looking into some
Hopefully this information can help you
