My ENT is VERY conservative on painkiller and initially encourage me only to use tylenol or motrin. Fortunately the radiation oncologist was more agressive and prescribed the hyrocodone. He gave me a dosage range because I was using the liquid so I started with one amount but was told I could keep increasing it which happened over time. I also took liquid morphine for a while and in my case (which was unusual) the pain got so bad and I became so ill that I was hospitalized and while in the hospital I was on an IV of dilaudid. I was on some very heavy doses for a while. I tell people that my last memory the beginning of June 2001 was standing over the kitchen sink screaming in pain because I couldn't swallow the pain medicine (and diflucan burned terribly also, I ended up with that by IV too). The next several months I describe as being in a drugged haze but it is the only way I could have gotten through it. I still had 2 weeks left of radiation when I was hospitalized.
Discuss your concerns with the doctor. Maybe your mother can be on a lower dose to start with and if that works, great or she can increase the dose as she needs. The treatment is wearing enough without serious pain also. Once I started feeling really bad I didn't care if I slept alot. I was annoyed that I had this enforced "vacation" and all the time to do nothing and nothing was all I had the energy to do!