Fetaine was one of the most feared biological weapons on Barrayar, a potent mutagen that not only damaged the victim, but all of the victim's potential offspring with serious genetic defects. There was almost no protection against the acidic substance due to its extremely high penetration value. The poison infected via inhalation or touch. It could be denatured via extreme heat.

The last known stores of fetaine were found on Kyril Island, which led to the Kyril Island Incident when some of the barrels leaked open after an accident.