I usually take 50 mg of amitryptiline. i might have accidentally taken 100mg. will that cause an overdose?

No: 100 mg is within the safe dosing range for this drug. If you experience any out of the ordinary symptoms, though, it would be prudent to give your prescriber a call.

Unlikely: Therapuetic dose for Amitriptyline is 25-150mg daily. You are clearly with in the therapeutic range. Onset of symptoms of an overdose typically occurs within 2 hours of ingestion, which corresponds to the peak ca serum level, which may range from 2-12 hours.