Ack! Don't do it. As many people have mentioned here, the side effects are awful. There's a reason that the dose is what it is, it gives the maximum benefit with the fewest side effects. I too have been obese and was on the prescription dose for some time, but even as desparate as I was to loose weight the side effects were not worth it.
Even knowing this, if anyone does decide to double it PLEASE PLEASE PLEASE, be under the close scrutiny of your doctor and stay home, it's no fun at all having that oil escape your body in public. You may feel just like you've let loose a puff of gas until you check your panties and they are not ruined (along with your jeans and any thing you've sat on). Panty liner? Sheesh, try a Depends instead.
My two cents, take it or leave it :-)
StillTammy