Flax contains a cyanogenic glycoside which, under certain conditions, can release cyanide. One of these conditions is an extremely acidic environment, pH below 1. While hydrochloric acid secreted by cells in the stomach does have a pH of 1, the presence of pepsin and other digestive enzymes raises the pH of the normal empty stomach to 3. The presence of food and water in the stomach will raise the pH even further. The function of the stomach is to mix and churn incoming food with HCl acid and stomach juices to begin the break-down and liquefying process. Absorption of anything in the food doesn't take place until it enters the small intestine. The normal pH of the duodenum is alkaline, pH 8 .. to neutralize the acid from the stomach. It is unlikely that the cyanogenic glycoside in flax seeds will release significant amounts of cyanide in the human digestive tract.

Other foods that naturally contain cyanogenic glycosides include:

• almonds
  cherries, peaches, apricots
  seeded grapes
  cassava
  sorghum
  corn
  alfalfa
  apples, pears
  beans
  coffee

While the glycosides are most concentrated in the seed, leaves and root of the plants, it is present to some degree in the flesh of the fruits as well.

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On the other hand ...

Cyanide gas does not require further processing in the body. It's already toxic, and enters the bloodstream rapidly when inhaled into the lungs. Common sources of cyanide gas include industrial waste spewing into our air, gases given off from the hot asphalt of city streets, and the burning end of a cigarette. There is more hydrogen cyanide in the second-hand smoke from ONE cigarette than the potential cyanide that could be yielded from the cyanogenic glycoside in a typical serving of flax.

Doreen