Interestingly, researchers suspect it's not just a matter of getting heavier women to take more of the Plan B chemical, called levonorgestrel. There have been no studies to check whether more levonorgestrel would work for heavier women, but, in general, the contraceptive scientists Popular Science spoke with thought that that is unlikely to solve the problem. (If you'd like to try anyway, levonorgestrel is safe to take in a double dose.) With levonorgestrel, what you put in isn't directly related to what you get out. In other words, when you take a double dose of levonorgestrel, the amount of the chemical that shows up in your bloodstream doesn't double. That's true no matter what you weigh.