You know, there are many types of marijuana. I don't know all the details, but there's Sativa, Indica and Hybrid, these are the main events.
And I don't know which one does what, but one type calms you physically and mentally and 20 minutes after that, you will fall asleep. That's the chemical reaction of a human body.
Then there's a different type, which increases your energy and therefore productivity, the exact opposite of the last one. It also depends on the amount of THC in any concrete joint.
To say what I acutally want to say, do you just want to smoke weed and lose weight? Or do you want to use it like a fuel that gives you energy to exercise. Because that's two different things.
It is also commonly known that cigarettes make you lose weight. Again, not in my case, I am not a cigarette/marijuana guru, but I know as much.
But really, you gotta ask yourself, is this the way you wanna go? Because in my opinion, there are much better ways, and MEANS, to lose weight, rather than marijuana/cigarettes/whatever.
If I may recommend something, find a good motivation to start losing weight. Motivation is the key, without it, it won't work. If you don't have a strong motivation, try something like DDP Yoga.