Untreated sinus infection 1.5 months. dr appt next week but have 500mg amoxicillin on hand (22 pills). would like to start treatment now with 3 per day until appt. wt 103lbs. will that harm me?

No: But you should always consult a doctor prior to retaking antibiotics.

Probably not.: If you have that much amoxicillin, is it because you didn't complete a full series of treatment before? If so, the Amoxicillin might be ineffective. The better course is to call the doctor, tell him/her that you are familiar with sinusitis, that you have these antibiotics, and would he/she recommend that you start them. When you are prescribed a course of antibiotics, take them all. Period.