Can you take Benadryl and fioricet together?

Answer 1

Yes you can, but if you're looking to potentiate, benadryl (Diphenhydramine HcL) only potentiates opiates mainly.
If the Fioricet isn't helping your headache and you are seeking benadryl to potentiate in order to achieve further effectiveness, you just need to talk to your prescribing doctor and/or GP to handle your headache/migraine/whatever the butalbital was scripted for.
If you are taking the benadryl simply to combat the effects of the caffeine in order to sleep, then just don't take more than one Fioricet after noon to 4pm. If insomnia persists, again.. talk to your doctor.
You can also request your doctor to script butalbital without caffeine (Phrenilin comes to mind, it does not contain caffeine. (See the links I have attached to this page.)

You can take benadryl WITH Fioricet, but there are complications(See the links I have attached to this page.) that have arisen and have been documented with this combination. Long story short, just talk to your doctor. He or she can give you an opinion calibrated to your current situation of which I do not have full knowledge. I have no idea why you need to take diphen.hcl alongside butalbital/Fioricet.