Clopidogrel (Plavix), a selective P2Y 12 receptor antagonist, is widely prescribed to reduce the risk of heart attack and stroke and acts via the inhibition of platelet aggregation. Accumulating evidence now suggests that extracellular nucleotides, signaling through P2 receptors, play a significant role in bone, modulating both osteoblast and osteoclast function. In this study, we investigated the effects of clopidogrel treatment on (1) bone cell formation, differentiation, and activity in vitro; and (2) trabecular and cortical bone parameters in vivo. P2Y 12 receptor expression by osteoblasts and osteoclasts was confirmed using qPCR and Western blotting. Clopidogrel at 10 mM and 25 mM inhibited mineralized bone nodule formation by 50% and >85%, respectively. Clopidogrel slowed osteoblast proliferation with dose-dependent decreases in cell number (25% to 40%) evident in differentiating osteoblasts (day 7). A single dose of 10 to 25 mM clopidogrel to mature osteoblasts also reduced cell viability. At 14 days, !10 mM clopidogrel decreased alkaline phosphatase (ALP) activity by 70% and collagen formation by 40%, while increasing adipocyte formation. In osteoclasts, !1 mM clopidogrel inhibited formation, viability and resorptive activity. Twenty-week-old mice (n ¼ 10-12) were ovariectomized or sham treated and dosed orally with clopidogrel (1 mg/kg) or vehicle (NaCl) daily for 4 weeks. Dual-energy X-ray absorptiometry (DXA) analysis showed clopidogreltreated animals had decreases of 2% and 4% in whole-body and femoral bone mineral density (BMD), respectively. Detailed analysis of trabecular and cortical bone using micro-computed tomography (microCT) showed decreased trabecular bone volume in the tibia (24%) and femur (18%) of clopidogrel-treated mice. Trabecular number was reduced 20%, while trabecular separation was increased up to 15%. Trabecular thickness and cortical bone parameters were unaffected. Combined, these findings indicate that long-term exposure of bone cells to clopidogrel in vivo could negatively impact bone health. ß