The health effects of eating expired food range from "none" to "severe." It all depends on what food you eat and when you eat it. For example, eating a carrot a day over its expired date usually won't pose health problems. However, eating a piece of pork three weeks past its best can cause an upset stomach and probably a serious case of food poisoning.
Expiry dates cover several different stages. For example, the sell-by date refers to the date when the store will discard the item. The best before date refers to the quality and taste of a product -- not necessarily the health effects if you consume it past that date. The use-by date refers to the last safe date before it starts to turn bad. However, this doesn't necessarily mean that the very next day the product is poisonous. It's a marker to show when the food degrades to a lower quality and a potentially dangerous state.
Though some expiry dates relate to product taste, in some cases eating expired food triggers food poisoning. Symptoms and health effects vary depending on the type of poisoning. However, common signs include cramping in the stomach area, frequent vomiting, fever, dizziness, dehydration and persistent diarrhea. The symptoms may last a few hours, days or weeks depending on the severity of the condition. Some food stored past its use-by date in poor conditions can even become contaminated with the serious bacterial infections salmonella or listeria.
One obvious sign of food well past its expiry date is the growth of mold. Mold includes several types of fungi. Their spores land on the food from the air and start to grow. Some molds produce toxic substances as they develop. For example, mycotoxins may grow on nuts, celery, apples, grapes and other food sources, according to the USDA. Mycotoxins can cause itchiness, nausea, dizziness and sometimes headaches.
While you should take care with any food past its expiry date -- particularly poultry and meat -- it's worth remembering that most dates are simply guidelines. Many exist to protect companies from potential lawsuits if you eat a bad piece of food and get sick. USC professor LaVonna Lewis said in an ABC article that in most cases food just past the expiry date poses no threat, though extra care should be taken with out of date food given to pregnant women and infants.