SummaryThis study assessed whether vitamin K, given with oral bisphosphonate, calcium and/or vitamin D has an additive effect on fracture risk in post-menopausal women with osteoporosis. No difference in bone density or bone turnover was observed although vitamin K1supplementation led to a modest effect on parameters of hip geometry.PurposeSome clinical studies have suggested that vitamin K prevents bone loss and may improve fracture risk. The aim was to assess whether vitamin K supplementation has an additive effect on bone mineral density (BMD), hip geometry and bone turnover markers (BTMs) in post-menopausal women with osteoporosis (PMO) and sub-optimum vitamin K status receiving bisphosphonate, calcium and/or vitamin D treatment.MethodsWe conducted a trial in 105 women aged 68.7[12.3] years with PMO and serum vitamin K1≤ 0.4 µg/L. They were randomised to 3 treatment arms; vitamin K1(1 mg/day) arm, vitamin K2arm (MK-4; 45 mg/day) or placebo for 18 months. They were on oral bisphosphonate and calcium and/or vitamin D. We measured BMD by DXA, hip geometry parameters using hip structural analysis (HSA) software and BTMs. Vitamin K1or MK-4 supplementation was each compared to placebo. Intention to treat (ITT) and per protocol (PP) analyses were performed.ResultsChanges in BMD at the total hip, femoral neck and lumbar spine and BTMs; CTX and P1NP did not differ significantly following either K1or MK-4 supplementation compared to placebo. Following PP analysis and correction for covariates, there were significant differences in some of the HSA parameters at the intertrochanter (IT) and femoral shaft (FS): IT endocortical diameter (ED) (% change placebo:1.5 [4.1], K1arm: -1.02 [5.07],p=0.04), FS subperiosteal/outer diameter (OD) (placebo: 1.78 [5.3], K1arm: 0.46 [2.23]p=0.04), FS cross sectional area (CSA) (placebo:1.47 [4.09],K1arm: -1.02[5.07],p=0.03).ConclusionThe addition of vitamin K1to oral bisphosphonate with calcium and/or vitamin D treatment in PMO has a modest effect on parameters of hip geometry. Further confirmatory studies are needed.Trial registrationThe study was registered at Clinicaltrial.gov:NCT01232647.Supplementary InformationThe online version contains supplementary material available at 10.1007/s11657-023-01288-w.