That's actually an *excellent* question for which I could not find a ready answer. The product is labeled for "outdoor" use, but there's no warning on the product that says not to use indoor.
(You can see the back of the package here:
http://www.ebay.com/itm/Esbit-Solid-Fue ... 2149632320)
You have three concerns: One review mentions a "slight odor" when burning, another mentions that the size of the flame is "uncontrollable" and burns at about 1300 degrees. The most important concern with any burning item is carbon monoxide - since this burns so fast, it would be hard to monitor.
If *I* were considering such a purchase, I would simply email the manufacturer and ask : )
https://esbit.de/en/service/#contactor
https://esbit.de/en/legal-notice/Hope that helps : )