Table 1.

Summary of the effects of synergists on pyrethroid absorption and metabolism

	Pyrethroids	Pyrethroids with piperonyl butoxide	Pyrethroids with organophosphorus (OP) compounds
Absorption	Dermal absorption poor; maximum 1.5%;3 when orally administered, 19–50% cypermethrin absorbed	Dermal absorption increases to approximately 8%;3 following ingestion 70% or more of the administered dose absorbed	Higher absorption of ingested dose
Metabolism	Rapid metabolism in the liver by oxidation, hydrolysis, and conjugation reactions3	Inhibition of CYP mono-oxygenase enzymes by up to 50% within 3 hours;3 does not return to normal until about 36 hours	OP inhibit carboxyesterases responsible for pyrethroid hydrolysis11