From what I understand the medication mainly makes it easier to stay in a calorie deficit. If protein stays high and lifting continues while fats and carbs drop, muscle retention should be good. The drug just quiets the hunger so the deficit happens. Without training or decent protein intake the body will use muscle for fuel along with fat.
I've gone through quite a few clinical trials and research papers on reta. Two different ideas keep coming up about how it affects the muscle-to-fat loss ratio. One view is that the ratio improves and you lose less lean tissue relative to fat than with ordinary dieting. The other view is that the ratio stays the same as normal. I know the talk about higher fat oxidation, but some say the effect is too small to change the actual numbers. I'm considering a modest recomp before summer and want to know which side is more likely.