One example of nidrei shegagot that do not need to be annulled (see daf 21) is someone who says “I take a neder to refrain from eating onions, since onions are bad for the heart.” Upon being informed that one type of onion – batzal kuferi – are good for the heart, the person will be permitted to eat all types of onions.
The potential danger involved in eating onions is described by the Gemara in Eiruvin (see Eiruvin 29) as relating to the leaves of the onion, which are potentially dangerous and cannot be used. In that Gemara a is brought which teaches that onions should not be eaten because of “the snake that is in it.” The continues with a story that Rabbi Hanina ate half an onion with half of the “snake” that was in it and became ill to the extent that he was close to death. His peers then prayed on his behalf and he recovered, since the generation needed his teaching and leadership.
The “snake” that the Gemara understands to be the danger lurking in the onion is subject to much speculation. The Ritva suggests that it is a worm that is found in the leaves of the onion that is potentially lethal. According to most traditions, however, it refers to a sprouting onion, which looks very much like a snake. It is difficult to come to a clear conclusion regarding the Gemara’s contention that eating onions generally, or their leaves specifically, presents a danger, since experience shows that onions are eaten with no ill effects. Nevertheless, onions contain the oil compound Allyl propyl disulfide (C3H5S2C3H7). Ingestion of even relatively small amounts of raw onions can, theoretically, cause toxicity that affects hemoglobin – the protein in red blood cells that carries iron, which binds to oxygen, which is carried through our bodies. People with specific sensitivity may even be in danger of death.