Since Fioricet/Codeine is a Schedule 3 drug (meaning not even close to the potency level of Fentanyl) it doesn't really affect the therapeutic levels. However, using anything that depresses the...
You probably won't notice it (the Schedule IV version of codeine, what most people get, is pretty weak compared to Fentanyl) but it and similar lower-end opiates are used for breakthrough meds with...