Science and Natural Product Safety

I said it in 2009, http://ktanihairsense.blogspot.ca/2009/12/natrual-products-for-hair.html
"Traditional use of a natural product does not signify its safety."

It still applies today. Tradition alone is not a test of product efficacy either. Topical use of natural products can affect health too, as toxic constituents can be absorbed in some cases by intact skin.

"Comfrey (Symphytum spp.)", 2013
The hepatoxic, carcinogenic "alkaloids may also be absorbed through intact skin" Note: Grade B uses require "further" or "additional" research to "confirm" results. See also this post.

"Hepatotoxic Botanicals - An Evidence-based Systematic Review", 2013 *Note: Margosa oil is Neem oil, bolding added by me
http://ejournals.library.ualberta.ca/index.php/JPPS/article/viewFile/17973/15135
"The hepatotoxicity of herbs was extensively acknowledged. ... Further scientific studies with high and good quality are needed to identify toxic compounds and understand the exact mechanism of hepatotoxicity-induced by herbs."
Table 1 lists the botanicals discussed in detail, Pages 402-404. Just: control f, type 402, and hit enter to access it quickly (you have to scroll up a bit to start at the Table beginning). A control f keyword or number search is a great way to find, or check if something you are looking for is in a long document. If what you are looking for is in the document more than once, hitting enter repeatedly will take you to all locations.
*Update: For neem, (aka margosa, Azadirachta indica, A. indica), See also this post and this one.

If you use natural henna, it may be a good idea to keep it safe from your pets, to protect their health.
"Hemolytic anemia after ingestion of the natural hair dye Lawsonia inermis (henna) in a dog.", 2013 http://www.ncbi.nlm.nih.gov/pubmed/24180624