In 2001 the United Kingdom Food Standards Agency found in testing various soy sauces manufactured in mainland China, Taiwan, Hong Kong, and Thailand (made from hydrolyzed soy protein, rather than being naturally fermented) that 22% of tested samples, contained a chemical carcinogen named 3-MCPD

Most varieties of soy sauce contain wheat, to which some people have a medical intolerance.[37] However, some naturally brewed soy sauces made with wheat may be tolerated by people with a specific intolerance to gluten because gluten is not detectable in the finished product.[38] Japanese tamari soy sauce is traditionally wheat-free, and some tamari available commercially today is wheat- and gluten-free.

https://en.wikipedia.org/wiki/Soy_sauce