If you drink milk long past the expiration date, you run the risk of developing food poisoning from unhealthy bacteria that might have grown in the milk. Food poisoning can be so mild you might not even realize you have it, but it can be severe in some cases. You might develop stomach cramps, nausea, diarrhea or a fever. It can last for hours or days, and you might start to feel the effects of the bad milk within a couple hours of drinking it.