Galphimine-B (G-B), a compound isolated from Galphimia glauca, has been shown to possess important anxiolytic activity. In this study, we evaluated the effectiveness and tolerability of a G-B standardized extract (experimental treatment) that was administered daily for 10 weeks in patients with moderate or severe Generalized Anxiety Disorder (GAD). Alprazolam was used as control treatment and administered under the same conditions. A total of 167 patients were included. At the start of the study, the severe anxiety condition prevailed, with an average on the Hamilton Anxiety Scale of 35.1 ± 8.8 and 35.8 ± 8.1 points in the control and experimental groups, respectively. After the 10 weeks of administration, the average was reduced in the control group to 4.6 ± 6.5 points and in the experimental group to 3.5 ± 5.5 points. Therapeutic success in the control group was 85.7% and in the experimental group, 92.0%. A high proportion of patients (22.2%) treated with Alprazolam manifested daytime sleepiness, while in the group treated with the G-B standardized extract, daytime sleepiness was found in 4.7%. In conclusion, a G-B standardized extract demonstrated therapeutic effectiveness in patients with GAD, without exhibiting significant difference with Alprazolam, but showing fewer cases of daytime sleepiness. The trial was registered at http://clinicaltrials.gov by identifier: NCT03702803.