Gastrointestinal nematodes affect the animal's health and cause economic losses in meat, milk, and wool production. Essential oils and their terpenoids have been shown to effectively control gastrointestinal nematodes and may be an alternative to control gastrointestinal nematodes. The great advantage of terpenoids is the possibility of acting on the parasite in a multidirectional way on the neuromuscular system and body structures of nematodes. The current chapter describes the pharmacological basis of the combination of terpenes and synthetic anthelmintics as an alternative for increasing antiparasitic efficacy. It is necessary to evaluate if these combinations show antagonist, additive or synergic effects at the pharmacokinetic and pharmacodynamic levels. The physicochemical properties, pharmacokinetic features and potential drug drug interactions at the metabolism or transport level of monoterpenes may be relevant for obtaining effective concentrations against different nematodes. In this context, the prediction of absorption, distribution, metabolism and excretion (ADME) is essential to optimize the anthelmintic action of these compounds. The rapid absorption and elimination of monoterpenes after their oral administration may directly influence the drug concentration level attained at the target parasites and the resultant pharmacological effect. Therefore, investigations on the dose schedule, administration route and type of pharmaceutical formulation are necessary. The integration of in vitro assays, in silico analysis, and in vivo pharmaco-parasitological studies are relevant to corroborate the kinetic/metabolic interactions and the efficacy of bioactive natural products combined with synthetic anthelmintics