I Am 63 Yrs Old. About 40 Lbs Overweight. I

I am 63 yrs old. about 40 lbs overweight. I am on hormones in which i feel great. i am very active and continue to gain weight. i do not eat a lot of sugar or drink sodas. i eat very little during the day but the evening meal i think my portion is too much. But i walk and bicycle every day. always on the go and i never eat after 6Pm. I started on bontril last week along with a diet plan. it seems to be working for me as far as the side effects go. too early to tell if i have lost any weight. but my question is, Is it the medication making you loose the weight or is it the diet. Why should i bother with Bontril if the diet causes you to loose weight. I just don't understand that.