The German Federal Institute for Risk Assessment has established a tolerable daily intake (TDI) of 0.1 mg coumarin per kg body weight, but also advises that higher intake for a short time is not dangerous. {1} Meanwhile, the Occupational Safety and Health Administration (OSHA) of the United States does not classify coumarin as a carcinogen for humans. {2}
There are two very distinct kinds of spices on the market which are commonly called "cinnamon". True cinnamon (derived from the plant Cinnamomum verum), also called Ceylon cinnamon and cassia cinnamon (derived from the plant Cinnamomum cassia). Although they are closely related they are different species and the distinction between the two was made early on since they are not only regionally separated but also tasting slightly different. And since your tongue might already tell you that: meaning their chemical composition is different. Concerning the quality of taste: true cinnamon is commonly described as milder, weaker, more elegant. Cassia on the other hand is cheaper per volume and more efficient to achieve a certain level of cinnamon taste.
I'll focus on cassia cinnamon rather than Ceylon ("natural cinnamon") because the levels of coumarin are higher and it's better to be safe than sorry.
Doing the math, take your weight (in lbs.) and multiply by .221 to get the daily recommended limit in milligrams (in Germany).
Ceylon Cinnamon has less than 0.04% Coumarin while Cassia Cinnamon has
in the region of 4%. {1}
Let's say you're petite or a developing child (80 lbs.) and can't handle as much coumarin. 80 * .221 = 17.68 milligrams of coumarin.
If we imagine 17.68 as 4% of some daily intake of cinnamon, that would be 442 milligrams or .442 grams of cinnamon daily.
Of course, if you happened to weigh twice as much, your intake would be twice that, and so on.
Also, if you want to take 6 grams of cinnamon per day, it seems that Ceylon is the better choice to prevent overexposure to coumarin, since you can take close to a hundred times more.
Disclaimers:
- Levels of coumarin in cassia cinnamon vary greatly even in bark from
the same tree. {4}
- The recommended daily intake exists in Europe, not US.
- Levels of coumarin from cassia are already too high from a health standpoint if eaten as a spice in comparatively low amounts, intake levels needed for a "natural supplement" of unproven benefit will quickly exceed that range
Sources:
http://www.bfr.bund.de/en/faq_on_coumarin_in_cinnamon_and_other_foods-8487.html
https://www.osha.gov/dts/chemicalsampling/data/CH_229620.html
http://www.sciencedaily.com/releases/2010/11/101103135352.htm