Pyrethrin refers to a class of organic compounds collectively known as pyrethrins that are derived from the parent chemical, pyrethrum. Although there are six major pyrethrins, the most common are a pair of esters distinguished from each other as pyrethrin I and pyrethrin II. Both of these pyrethrins possess a base cyclopropane molecular structure. However, pyrethrin I has one less oxygenated carbon molecule than pyrethrin II.

Generally speaking, pyrethrin is the active constituent of the flowers of the pyrethrum plant, better known as chrysanthemum. A member of the daisy family, several related species are grown as perennial ornamentals. However, pyrethrin is obtained from either the Dalmatian chrysanthemum (Chrysanthemum cinerariaefolium) or the Persian chrysanthemum (C. coccineum), also known as Painted Daisy. These two species are grown commercially to produce pyrethrin to be used as an insecticide. While other chrysanthemums also possess insecticidal properties, they are not as effective.

Pyrethrin is produced from the flowers of the plant by one of two methods. The flowers are dried and ground into pyrethrum powder, or they are subjected to solvents to extract the volatile oils to produce a resin. Pyrethrin is sold under several different trade names, which include Firmotox, Buhach, Ofirmotox, Parexan and NA 9184.

The species from which pyrethrin is obtained are considered excellent insect deterrents even in their natural state. This is because they exhibit protective effects on neighboring plants and protect them from infestation. In fact, these plants are often featured in companion planting, a method of natural pest control used by organic gardeners.

Pyrethrin is classified as a neurotoxin. It quickly permeates the shell or skin of the insect and causes immediate paralysis. However, in the event that a large area or a large number of insects are being treated, it may only be effective as a "knockdown" measure. In fact, the insect’s protective enzymes will degrade the insecticide and permit recovery. For this reason, pyrethrin may be administered in combination with organophosphates or carbamates to deter enzyme activity and ensure a lethal dose.

Pyrethrins are considered relatively nontoxic to mammals, including humans. However, pyrethrin I is deemed a "Restricted Use Pesticide" by the United States Environmental Protection Agency and its availability and use is limited to certified applicators. This may be due to the fact that pyrethrins are highly toxic to fish, and moderately toxic to birds and bees. Pyrethrin compounds do not persist in the environment since they readily biodegrade when exposed to oxygen, moisture and sunlight.