Cypermethrin (CP) is a synthetic pyrethroid used as an insecticide in large-scale commercial agricultural applications as well as in consumer products for domestic purposes. It behaves as a fast-acting neurotoxin in insects. It is easily degraded on soil and plants but can be effective for weeks when applied to indoor inert surfaces. It is a non-systemic and non-volatile insecticide that acts by contact and ingestion, used in agriculture and in pest control products. Exposure to sunlight, water and oxygen will accelerate its decomposition. Cypermethrin is highly toxic to fish, bees and aquatic insects, according to the National Pesticide Information Center (NPIC) (previously National Pesticides Telecommunication Network) in the USA. It is found in many household ant and cockroach killers, including Raid, Ortho, Combat, ant chalk, and some variants of Baygon in Southeast Asia.

Cypermethrin is used in agriculture to control ectoparasites which infest cattle, sheep, and poultry.

Cypermethrin is moderately toxic through skin contact or ingestion. It may cause irritation to the skin and eyes. Symptoms of dermal exposure include numbness, tingling, itching, burning sensation, loss of bladder control, incoordination, seizures and possible death. Excessive exposure can cause nausea, headache, muscle weakness, salivation, shortness of breath and seizures.^{[citation needed]}

Pyrethroids may adversely affect the central nervous system. Human volunteers given dermal doses of 130 μg/cm² on the earlobe experienced local tingling and burning sensations. One man died after eating a meal cooked in a 10% cypermethrin/oil mix that was mistakenly used for cooking oil. Shortly after the meal, the victim experienced nausea, prolonged vomiting, stomach pains, and diarrhea which progressed to convulsions, unconsciousness and coma. Other family members exhibited milder symptoms and survived after hospital treatment.

It may cause allergic skin reactions in humans.

In humans, cypermethrin is deactivated by enzymatic hydrolysis to several carboxylic acid metabolites, which are eliminated in the urine. Worker exposure to the chemical can be monitored by measurement of the urinary metabolites, while severe overdosage may be confirmed by quantitation of cypermethrin in blood or plasma.

Cypermethrin is very toxic to cats which cannot tolerate the therapeutic doses for dogs. This is associated with UGT1A6 deficiency in cats, the enzyme responsible for metabolizing cypermethrin. As a consequence, cypermethrin remains much longer in the cat's organs than in dogs or other mammals and can be fatal in large doses.

In male rats cypermethrin has been shown to exhibit a toxic effect on the reproductive system by Elbetieha et al. 2001. In another result, after 15 days of continual dosing, both androgen receptor levels and serum testosterone levels were significantly reduced. These data suggested that cypermethrin can induce impairments of the structure of seminiferous tubules and spermatogenesis in male rats at high doses.