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http://advantagemulti.petparents.com/howItWorks.cfm

Advantage MultiTM (imidacloprid + moxidectin) Topical Solution is to be administered monthly, and when applied directly to your pet's skin, prevents heartworm disease, kills adult fleas and controls flea infestations, and treats and controls intestinal worms. The two active ingredients in Advantage Multi are imidacloprid and moxidectin. Imidacloprid has a high affinity for the nicotinic acetylcholine receptors in the post-synaptic region of the nervous system of fleas. The ensuing inhibition of cholinergic transmission in insects (i.e., fleas) results in paralysis and death. Moxidectin interacts with gaba-amniobutyric acid (GABA) and glutamate-gated chloride channels. This leads to opening of the chloride channels on the post-synaptic junction, the inflow of chloride ions and induction of an irreversible resting state. The result is flaccid paralysis of susceptible parasites, followed by their death and/or expulsion.