Many of the active ingredients in flea and tick powders are harmful to humans, or suspected of being carcinogenic, or have effects on the nervous system.
The Humane Society has a page on OTC flea and tick products, one of the sections details a couple of these effects:
Besides pyrethroid-based products, ingredients to be wary of are organophosphate insecticides (OPs) and carbamates, both of which are found in various flea and tick products. The only OP currently found in flea and tick products in the U.S. is tetrachlorvinphos. This chemical is classified by the EPA as being "likely to be carcinogenic to humans." There are questions about the effects of long-term, cumulative exposures as well as combined exposures from the use of other products containing OPs and carbamates. Permethrin is another chemical that the EPA has classified as "likely to be carcinogenic to humans" if ingested orally.
If the ingredient list includes carbaryl or propoxur, the product contains a carbamate. According to the NRDC, the potential dangers posed by thee products are greatest for children and pets. Propoxur is considered to be a "probable human carcinogen" by the EPA. As of September 2010, Carbaryl will no longer be permitted for use in new pet products. However, existing stock of flea/tick products containing carbaryl can still be sold. The HSUS recommends that pet products containing carbaryl should be disposed of and not used on pets.
Additionally, one of the current varieties of powder (K9 Advantix II) has the following warning on it's website for the product:
Hazards to Humans: Warning. Causes substantial but temporary eye injury. Do not get in eyes or on clothing. Harmful if swallowed. Harmful if absorbed through skin. Avoid contact with skin. Wash thoroughly with soap and water after handling and before eating, drinking, chewing gum, using tobacco or using the toilet. Remove and wash contaminated clothing before reuse.
So there are a host of possible side effects, from possible nerve damage up through it being a possible carcinogen. I would not advise using them on yourself.