OBJECTIVES: To assess medication adherence and switching rates of testosterone replacement therapy (TRT) in the US.  METHODS: We conducted a retrospective claims database study using the MarketScan® Commercial database from January 2005 through December 2011. The study included men 18-65 years of age who had initiated TRT and were diagnosed with hypogonadism or hypogonadotropic hypogonadism (ICD-9-CM: 257.3 and 257.4). A minimum of 6 months continuous enrollment before and 12 months after the TRT index date was required. Adherence was measured by the Mediation Possession Ratio (MPR) and the Proportion of Days Covered (PDC), with the adherence rate defined as MPR≥0.8. The rate of the first switching was summarized by TRT formulation.  RESULTS: Of 106,039 patients with hypogonadism, the mean MPR and the mean PDC of any TRT during the 360 days study period was 0.47 and 0.44, respectively. The overall adherence rate over 12 months was 21.4% ranging from 28% (pellets), 19.9% (SAIs), 17.7% (gels), 7.8% (buccal), and 6.6% (patch), respectively. Similarly, the average PDCs were significantly higher for pellets (0.59) compared to gels (0.43), SAIs (0.39), buccal (0.28), and patch (0.27) (P<0.001).  About 13% (n = 13,577) of patients switched from their initial therapy to a different TRT formulation. Patients starting with patch were most likely to switch to a different formulation (39.3%), followed by patients with SAIs (12.1%), pellets/buccal (12.0%), and gels (10.3%).  CONCLUSIONS: Our study showed low adherence rates of testosterone replacement therapy among hypogonadism patients. The adherence for patients starting on the long-acting testosterone formulation such as pellets was higher than those with other formulations. Patients starting with a patch demonstrated the highest switching rate compared to other formulations.