Rats fed high doses (1,000 milligrams per kilogram of body weight or millograms/kilogram) of pyrethrins (natural versions of pyrethroids) showed liver damage. There is also evidence that pyrethroids can harm the thyroid.
Pyrethroids, including sumithrin, have been shown in the lab to disrupt the endocrine system by mimicking the effects of the female sex hormone estrogen. In men, endocrine disruptors can lower the sperm count, and in women they can cause the growth of abnormal breast cells.
Piperonyl butoxide has been classified by the Environmental Protection Agency as a possible human carcinogen.