BackgroundObesity and anxiety are often linked but the direction of effects is not clear.MethodUsing genetic instrumental variable (IV) analyses in 5911 female participants from the Nurses' Health Study (NHS, initiated 1976) and 3697 male participants from the Health Professional Follow-up Study (HPFS, initiated 1986), we aimed to determine whether obesity increases symptoms of phobic anxiety. As instrumental variables we used the fat mass and obesity-associated (FTO) gene, the melanocortin 4 receptor (MC4R) gene and a genetic risk score (GRS) based on 32 single nucleotide polymorphisms (SNPs) that significantly predict body mass index (BMI). ‘Functional’ GRSs corresponding with specific biological pathways that shape BMI (adipogenesis, appetite and cardiopulmonary) were considered. The main outcome was phobic anxiety measured by the Crown Crisp Index (CCI) in 2004 in the NHS and in 2000 in the HPFS.ResultsIn observational analysis, a 1-unit higher BMI was associated with higher phobic anxiety symptoms [women:β = 0.05, 95% confidence interval (CI) 0.030–0.068; men:β = 0.04, 95% CI 0.016–0.071). IV analyses showed that BMI was associated with higher phobic anxiety symptoms in theFTO-instrumented analysis (p = 0.005) but not in the GRS-instrumented analysis (p = 0.256). Functional GRSs showed heterogeneous, non-significant effects of BMI on phobic anxiety symptoms.ConclusionsOur findings do not provide conclusive evidence in favor of the hypothesis that higher BMI leads to higher levels of phobic anxiety, but rather suggest that genes that influence obesity, in particularFTO, may have direct effects on phobic anxiety, and hence that obesity and phobic anxiety may share common genetic determinants.