What are the side effects of eating moldy bread?

Eating moldy bread can lead to food poisoning from mycotoxin or bacterial contamination. Most of the time it is not dangerous, though eating moldy food of any kind should be avoided.

Mycotoxin Poisoning
Mycotoxins are fungal poisons that are typically eaten, leading to mycotoxicoses, which can be fatal is left untreated. Examples are rare, such as in the year 857 A.D. when thousands of people died of ergotism from ingesting rye, which is commonly used in bread.

Bacterial Contamination

Bacterial contamination is the more common of the two side effects of eating moldy bread. It is commonly called food poisoning, which can lead to vomiting and diarrhea. In severe cases, food poisoning can also be fatal.