Avocados
The leaves, bark, skin, and pit of avocados contain a toxic fatty acid called persin, which also leeches into the flesh of the fruit from the pit and can be quite toxic to rats as well as other animals. For this reason, avoid feeding avocados as well as guacamole to rats.
Blue Cheese
What makes blue cheese blue is a type of mold (penicillium) which can be toxic to rats.
Carbonated Drinks
While the sugar alone is not very good for rats, rats are one of the few animals who cannot burp and therefore if they consume a larger amount of a soft drink they may get uncomfortable gas pain.
Chocolate
Though small doses of chocolate (especially milk chocolate) is all right, and can actually help with breathing problems, larger doses of chocolate, especially baker's chocolate and dark chocolate, can cause theobromine poisoning, which causes nausea, diarrhea, and increased urination, which can escalate to seizures, heart attacks, and eventually death.
Dried Corn
Can contain fungal contaminants such as aflatoxin and fumonisin which can cause liver or other cancers in rats. Fresh corn is all right in small doses. Also, if you feed laboratory blocks or other manufactured rat foods, choose brands where corn is not the main ingredient.
Dry Beans
Contains protease inhibitors that block the action of enzymes needed to digest protein.
Green Bananas
Contains a resistant starch, which is difficult to digest and can ferment in the large intestine.
Green Potato Skins and Tubers
Potatoes produce a toxin called solanine, a poison found in the deadly nightshade plant. Potatoes and tubers that are exposed to light turn green and begin producing solanine to help prevent them from being eaten while they grow. This toxin causes gastrointestinal and neurological distress, and can eventually lead to death if enough is eaten.
Licorice
Licorice plants and licorice candies contain glycyrrhizinic acid and glycyrrhetinic acid, which can increase blood pressure and be toxic to the liver and cardiovascular system in larger doses. I have also read on many rat websites that it contains a 'suspected neurotoxin' but have not found any articles to back this up, if anyone has any information on this please email me.
Orange Juice
Contains a hydrocarbon called D-Limonene, which causes renal cancer in male rats. It is fine for female rats, however. Oranges themselves are all right as long as you do not allow a male rat to consume any of the peel or rinds.
Peanut Butter
Can be very sticky and if the rat eats it too fast they can choke. Small, easily managed bits are fine as an occasional snack. Spreading it on a cracker can also make it easier for them to eat.
Raw Sweet Potato
Contains a trypsin inhibitor, which can block the digestion of proteins. I have also read that it contains cyanide forming compounds. Cooked sweet potato is fine.
Raw Red Cabbage
Contains thiaminase, an anti-nutrient enzyme that splits the thiamin molecule so that the vitamin is no longer nutritionally useful, and can cause a vitamin B deficiency.
Raw Peanuts
Contains protease inhibitors that block the action of enzymes needed to digest protein.
Soy Milk
Soy milk should not be given to rats who have tumors, as it contains precursors of estrogens and if the rat's tumor is growing for a hormonal reason it could make the tumor grow faster. Fermented soy milks are still fine, and any soy milk is all right for rats without tumors, though it may be a good idea to avoid soy products altogether just in case.
