Is chitosan toxic?

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Chitosans are considered to be non-toxic. Their cytocompatibility has been proven against a wide range of human cells, they are approved as food and feed additives and for purification of drinking water, and they are registered as "basic substance" in the EU. Even though they have antimicrobial activities, they do not kill bacteria or fungi - i.e. they are not bactericidal or fungicidal - but they rather inhibit microbial growth - i.e. they are bacteriostatic and fungistatic. The situation is a little less clear for nano-formulated chitosans as these appear to be easily taken up by human cells, e.g. by the epithelial cells of the lung, but are then not so easily degraded by the human chitinases. While toxicity of chitosan nanoparticles has not been shown, absence of evidence is not evidence of absence.