I'm very confused about what n how half life.works.I asked my pharmacist who is fantastic n I asked my dr also. This is how they explained it to me. If a drug has a half life of 12 hours and just say the drug is a 2mg xanax then after 6 hours 1 mgs of the drug is still working in ur body and the other 1 mg has been eliminated and is no longer working at all in ur body.then 6 hours later the other remaining 1 mg is gone and not working in your body.so let's say u take a emg xanax @ 12 noon and the half life is 12 hours then at 12 midnite 1 mgs has been eliminated and there is still 1 mg still working in ur body then after 6 more hours which would be 12 noon the next day the final 1 mg is gone and the drug is out of ur system completly. I want to make sure this is right. And I also want to thank everyone here for all the support u have given me here with my recent taper off fioricet.u guys have been amazing.my last fioricet was taken monday at 12 noon and the half life of the active ingrient infioricet is butalbital and it has a half life of 35 hours.I took my last fioricet @ 12 noon on monday august 1st today is thursday august 4th it is 3 00 now. I want to no if the fioricet is out of my system yet? As many of you no I have had horrible fears about having withdrawals after my taper.getting sober is not easy @ all.staying sober is even harder.I'm 35 I have been battling addictions since I was 20 yrs old I'm 35 now. Its a horrible disease! I also suffer from very bad anxiety.thanks for listening.