Eprinomectin is a member of the macrocyclic lactone class of endectocides. Compounds of the class bind selectively and with high affinity to glutamate-gated chloride ion channels which occur in invertebrate nerve or muscle cells. This leads to an increase in the permeability of the cell membrane to chloride ions with hyperpolarization of the nerve or muscle cell, resulting in paralysis and death of the parasite.

Compounds of this class may also interact with other ligand-gated chloride channels, such as those gated by the neurotransmitter gamma-aminobutyric acid (GABA).

The margin of safety for compounds of this class is attributable to the fact that mammals do not have glutamate-gated chloride channels; the macrocyclic lactones have a low affinity for other mammalian ligand-gated chloride channels, and they do not readily cross the blood-brain barrier.

In cattle, following subcutaneous administration, the bioavailability of eprinomectin is about 89%. The maximal mean plasma concentration of 58 µg/L was reached after 36-48 h.

In lactating sheep, the maximal mean plasma concentration of 19.5 µg/L was reached 33.6 hours after subcutaneous administration. The area under the curve mean value over a period of 7 days after dose injection was 73.3 µg*day/L. In non-lactating sheep, the maximal mean plasma concentration of 11.3 µg/L was reached after 26.7 hours after dose administration. The area under the curve mean value over a period of 7 days after treatment was 42.5 µg*day/L

In goats, the maximal mean plasma concentration of 20.7 µg/L was reached 36 h after administration. The area under the curve mean value over a period of 7 days was 66.8 µg*day/L.

There is a linear relationship between the dose administered and plasma concentration observed in the therapeutic dose range from 0.1 to 0.4 mg/kg.

Eprinomectin is highly bound (greater than 99%) to plasma proteins.

Eprinomectin is not extensively metabolised. Metabolites amount to approximately 10% of the total residues in plasma, milk, edible tissues and faeces.

In cattle, eprinomectin is eliminated with a half-life of 65-75 h and the major route of elimination is via faeces.

In sheep, eprinomectin is eliminated with a comparable half-life of 62-78 h.

In goats, eprinomectin is eliminated with a half-life of 91 hours.

Like other macrocyclic lactones, eprinomectin has the potential to adversely affect non-target organisms. Following treatment, excretion of potentially toxic levels of eprinomectin may take place over a period of several weeks. Faeces containing eprinomectin excreted onto pasture by treated animals may reduce the abundance of dung feeding organisms which may impact on the dung degradation. Eprinomectin is very toxic to dung fauna and aquatic organisms, is persistent in soils and may accumulate in sediments.