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Background Low bone mineral density (BMD) may constitute an underestimated comorbidity in schizophrenia patients undergoing long-term antipsychotic treatment. Glucagon-like peptide 1 (GLP-1) receptor agonists are antidiabetic drugs, which may also affect bone turnover. The current study comprises planned secondary analyses of the 'TAO study': Treatment of antipsychotic-associated obesity with a GLP-1 receptor agonist. The TAO study was an investigator-initiated, double-blind, randomized, placebo-controlled trial, investigating the effects of three months treatment with the GLP-1 receptor agonist exenatide 2 mg once-weekly in chronic obese, antipsychotic-treated patients with schizophrenia spectrum disorder. Methods First, we compared baseline bone turnover markers (BTMs) of 45 chronic, obese, antipsychotic-treated patients with the Danish Health2006 study cohort as reference population, and we calculated bone mineral density (BMD) T- and Z-scores. Second, investigated effects of three months of GLP-1 receptor agonist exenatide 2 mg once-weekly (n=23), or placebo (n=22) on BTMs and BMD in these patients. Data were initially analyzed without covariates by two-way repeated measures ANOVA. All analyses were repeated with mean prolactin level as a covariate to evaluate the potential effect of prolactin. Results In women (n=24), all baseline BTM measurements of procollagen type I N-terminal propeptide (PINP) and C-terminal cross-linking telopeptide of type I collagen (CTX) were within reference values. In men (n=21), 5% displayed lower PINP and 14% displayed lower CTX. One patient displayed BMD Z-score <-2, and 23% of patients (17% of women and 29% of men) displayed -2.5