To the best of my ability to determine, Promeris causes about 1 in 1000 cases of SEVERE skin drug reactions like PEMPHIGUS or EM or TEN.
This risk is too high if using the product for routine flea and tick control; resulting in about 2 cases each year; BAD!
For treating Demodex (every 2 weeks), this adverse event rate is more acceptable; resulting in about 1 case every few years.