Priapism is painful, and when it occurs it is an emergency, but this only happens in one out of 10,000 men with dosage of over 50mgm, usually higher. There are warning signs usually like painful erections that would lead you to stop using the drug immediately before priapism( prolonged painful erection of 3 hours or more.)
I can't tell you where to look for drug interactions, but if you browse around this site you will probably find one.