[3] It is also used to kill head lice in humans, but studies conducted in Paris and the United Kingdom have shown widespread resistance to phenothrin.

[3] It has increased risk of liver cancer in rats and mice in long-term exposure, with doses in the range of 100 milligrams per kilogram of body weight per day, or above.

Phenothrin has been found to possess antiandrogen properties, and was responsible for a small epidemic of gynecomastia via isolated environmental exposure.

[3] In 2005, the U.S. EPA cancelled permission to use phenothrin in several flea and tick products, at the request of the manufacturer, Hartz Mountain Industries.

[7][8] The products were linked to a range of adverse reactions, including hair loss, salivation, tremors, and numerous deaths in cats and kittens.