Terbutaline is a beta-adrenergic receptor agonist. In vitro and in vivo pharmacologic studies have demonstrated that terbutaline exerts a preferential effect on beta2-adrenergic receptors. While it is recognized that beta2-adrenergic receptors are the predominant receptors in bronchial smooth muscle, data indicate that there is a population of beta2-receptors in the human heart, existing in a concentration between 10% to 50%. The precise function of these receptors has not been established (see WARNINGS). Controlled clinical studies in patients given terbutaline subcutaneously have not revealed a preferential beta2-adrenergic effect. The pharmacologic effects of beta-adrenergic agonists, including terbutaline, are at least in part attributable to stimulation through beta-adrenergic receptors of intracellular adenyl cyclase, the enzyme which catalyzes the conversion of adenosine triphosphate (ATP) to cyclic 3’,5’-adenosine monophosphate (cAMP). Increased cAMP levels are associated with relaxation of bronchial smooth muscle and inhibition of release of mediators of immediate hypersensitivity from cells, especially from mast cells. Controlled clinical studies have shown that terbutaline relieves bronchospasm in acute and chronic obstructive pulmonary disease by significantly increasing pulmonary flow rates (e.g., an increase of 15% or more in FEV1). After subcutaneous administration of 0.25 mg of terbutaline, a measurable change in expiratory flow rate usually occurs within 5 minutes, and a clinically significant increase in FEV1 occurs within 15 minutes. The maximum effect usually occurs within 30 to 60 minutes, and clinically significant bronchodilator activity may continue for 1.5 to 4 hours. The duration of clinically significant improvement is comparable to that observed with equimilligram doses of epinephrine.  
Preclinical  
Studies in laboratory animals (minipigs, rodents, and dogs) have demonstrated the occurrence of cardiac arrhythmias and sudden death (with histological evidence of myocardial necrosis) when beta-agonists and methylxanthines are administered concurrently. The clinical significance of these findings is unknown.  
Pharmacokinetics  
Subcutaneous administration of 0.5 mg of terbutaline to 17 healthy, adult, male subjects resulted in mean (SD) peak plasma terbutaline concentration of 9.6 (3.6) ng/mL, which was observed at a median (range) time of 0.5 (0.08 to 1) hours after dosing. The mean (SD) AUC (0 to 48) and total body clearance values were 29.4 (14.2) hr • ng/mL, and 311 (112) mL/min respectively. The terminal halflife was determined in 9 of the 17 subjects and had a mean (SD) of 5.7 (2) hours. After subcutaneous administration of 0.25 mg of terbutaline to two male subjects, peak terbutaline serum concentrations of 5.2 and 5.3 ng/mL were observed at about 20 minutes after dosing.

Elimination half-life of the drug in 10 of 14 patients was approximately 2.9 hours after subcutaneous administration, but longer elimination half-lives (between 6 to 14 hours) were found in the other 4 patients. About 90% of the drug was excreted in the urine at 96 hours after subcutaneous administration, with about 60% of this being unchanged drug. It appears that the sulfate conjugate is a major metabolite of terbutaline and urinary excretion is the primary route of elimination.