The livers of certain animals are more dangerous to eat than others. The liver is very high in vitamin A and vitamin B12. Eating liver was the first treatment ever used for treating a deficiency of extrinsic factor, now known as vitamin B12.
I think it’s polar bear liver that is extremely high in vitamin A, which can cause hypervitaminosis A. Hypervitaminosis A is a toxic increase in vitamin A which can cause certain dangerous symptoms like liver damage, bone damage, and blurred vision.
Eating too much of any animal liver can cause hypervitaminosis A, but most people do not eat liver very often so it isn’t particularly common. According to Wikipedia, liver generally contains 813% of your daily supply of vitamin A in a single 100g serving.
100g of liver (probably pork or cow liver) also contains 1083% of your daily vitamin B12. Imagine eating only 100g of liver at once! I can’t!
The liver of pufferfish, commonly known as fugu contains the toxic chemical tetrodotoxin which can cause paralysis and death if eaten in sufficiently high (actually quite small) amounts.
The toxin also occurs in high amounts in the fish’s ovaries. Tetrodotoxin works by blocking voltage-gated sodium ion channels in the brain and is classified as a neurotoxin. Because of this mechanism, it also can cause numbness.
Ironically, the liver and ovaries of pufferfish are said to be the most tasty parts of the fish. Some people willingly eat small amounts of pufferfish liver at parties. It is said to make your lips numb and tingly, a subjectively pleasant feeling, because of the tetrodotoxin in the fish.
Vitamin A is very important for your vision and a variety of other bodily functions. It is an essential part of your diet, however as a fat-soluble vitamin, an excess of it can be toxic. Vitamin A allows to proteins rhodopsin and iodopsin to function in the eye, which are important for your vision.
A deficiency of vitamin A can cause night blindness, which is difficulty seeing at night. Vitamin A deficiency is common in some parts of the world, especially some places in Southeast Asia. A genetically modified form of rice has been developed called golden rice to increase vitamin A in deficient people’s diets.
Vitamin B12 is an important coenzyme that helps the enzymes methyl malonyl-CoA mutase and methionine synthase function. The first enzyme is an important enzyme in the Krebs cycle in the mitochondria while the second synthesizes the amino acid methionine and is part of the folate cycle.
In the end, pork and beef liver are not particularly dangerous to eat, unless you start eating it literally every day. I can’t really imagine someone doing that very often, liver has a very strong flavor and it’s not something many people are in the mood for very often.