Is it dangerous to eat moldy bread if you have a penicillin allergy?

No: Most mold on bread is rhizopus, not penicillium, and would not cause a reaction in those with allergy to the drug penicillin. There have been a few penicillium molds producing penicillin found in some sausages and cheeses, but generally, there probably is not enough penicillin to cause a reaction, since this is almost never seen. The usda recommends that moldy breads be discarded for everyone.

Unlikely: But why would you want to eat moldy bread? Just kidding. The basic answer is "no" but I am sure it is a possibility.