A synergist such as those used in veterinary and agricultural parasiticidal products are chemical compounds that enhance the efficacy of several active ingredients or help to overcome some types of resistance of external parasites to certain active ingredients. Synergist are not themselves active ingredients: alone they will have no effect on the parasite. They are often mixed with active ingredients such organophosphates, carbamates, synthetic pyrethroids and other natural insecticides. There are so far no synergists used in combination with anthelmintics.

Synergists in Veterinary Parasiticides

The synergist most widely used in veterinary parasiticides is piperonyl butoxide (= PBO), but there are other synergists used in agriculture, e.g. DEF (S,S,S-tributyl phosphorotrithioate, = tribufos) or N-octyl bicylcoheptene dicarboximide.

Molecular structure of piperonyl butoxide (PBO)

PBO is an inhibitor of cytochrome P450, a molecule that acts on many mechanisms that break down insecticides inside the parasite's body. By blocking this molecule the concentration of the active ingredient inside the parasite's body gets higher and/or remains effective for a longer period. In either case the active ingredient is more effective than without the synergist.

Some resistance mechanisms consist precisely in an enhanced detoxification or faster break down of the active ingredient inside the parasite. PBO can slow down this detoxification and this way reduce or even overcome the parasite's resistance.

The effect of each synergist depends on the specific active ingredient it is mixed with, as well as on the parasite species concerned. It works well with certain parasites (e.g. mosquitoes) and less with other ones. In resistant parasites with other resistance mechanism than the enhanced detoxification synergists may not help at all.

Synergists are widely used in household pesticides such as sprays, aerosols, foggers and the like against mosquitoes and many other household pests (cockroaches, ants, etc.). They are widely used in agriculture as well.

Use on veterinary parasiticides is moderate, mainly in soft pet products (shampoos, soaps, sprays, etc.) and in some pour-ons for livestock and horses against horn flies and sheep lice.

Synergists are usually considered as non toxic at the therapeutic concentrations and normally they don't pose an additional health hazard for animals, operators or pet owners. The toxicity enhancing effect of the synergist applies only to the parasite and not to the host.