Does anyone know if prescription diet pills (if so, which ones?) or those B-12 injections really work?

I am 187 lbs., 6’, & size 14. I was a skinny kid & always have had healthy eating habits & get plenty of exercise. However, for some reason around high school I started packing on a lot of weight, even though I ate healthy (lots of greens and vegetables and lean meats, never been too fond of carbs) & played field hockey, soccer, & lacrosse, as well as surfed. I’m only 20 years old, but I want to nip this in the bud. I am currently getting tested for a thyroid problem, but if that comes back negative I’m not sure what other options I have. I got off birth control as well. I’m so frustrated, if you know of any success stories please let me know.