How common is adverse reactions to denture impression material? anaphylaxis? how often do they happen

Never known: 35 years practicing ,30 as Prosthodontist, and never heard of an incident. Should not be a concern. Especially with today's materials. Good luck

Contact stomatitis: Some new study suggests that Polyether denture impression materials may evoke type IV allergic reactions. The number of such allergic reactions is very low. Good luck.

RARE: RARE, if at all. In over 40 years of practice, I have never personally seen or heard of this occurrence. If you have any known allergies, discuss this subject with your own dentist or allergist.