Severe issues like obstructive sleep apnea also make you a strong candidate
B12 from food and supplements has to pass through a fairly involved process in the stomach and gut before your body can use it, and that process becomes less efficient with age, with certain medications, and with diets that include little or no animal protein
Dakal TC, Solieri L, Giudici P (2014) Adaptive response and tolerance to sugar and salt stress in the food yeast Zygosaccharomyces rouxii
If you are losing weight consistently, tolerating the medication well, and progressing toward your goal, there is no clinical requirement to switch
Effects of low-dose, controlled-release, phentermine plus topiramate combination on weight and associated comorbidities in overweight and obese adults (CONQUER): a randomised, placebo-controlled, phase 3 trial