Cancer cachexia causes anorexia and metabolic disorders, eventually leading to sarcopenia, which in turn contributes to the development of functional disabilities. Although anamorelin hydrochloride tablets are marketed to treat cancer cachexia, their efficacy varies significantly among patients. Here, we investigated the efficacy of anamorelin and the factors associated with weight gain. The factors that contributed to weight gain in patients before starting anamorelin were as follows: the patients' disease stage had not progressed to refractory cachexia based on the cancer cachexia classification of the European Palliative Care Research Collaborative; the patients had received fewer lines of anticancer treatment at the start of oral administration of anamorelin; and the patients had not met all the criteria for starting treatment with anamorelin, namely, C-reactive protein level >0.5 mg/dL, hemoglobin level <12 g/dL, and albumin level <3.2 g/dL. These results suggest that early administration of anamorelin hydrochloride tablets may increase the response rate when cancer cachexia is diagnosed.