I am afraid that there is little you can do in this situation. It depends mainly on the level of exposure. The difficulty is that THC, the active ingredient of cannabis, gets stored up in fat and is released slowly into the blood. Cannabis, or rather, THC, takes a week to six weeks to clear from the body. For a mild exposure, say, a day of moderate smoking, a week would be sufficient to clear up the system. But for chronic exposure, in heavy smokers, 6 weeks would ideally be required for the cannabis to be cleared. There are some controversies, some researchers say that 4 weeks and not 6, is the cutoff for even heavy smokers. But it is better to assume 6 weeks to be on the side of safety. You can look up here: http://homepages.poptel.org.uk/DrDrew/testing.html Also look up here:
However, once the cannabis is lodged in the fat, and if you are a heavy smoker, its bound to be, then there's nothing which will clear it off. Although many products are advertised on the net and elsewhere, none of these are of any use. You can make this out from the fact that if they were of use, no one would ever be detected as positive. I am afraid I must tell you that there is practically nothing you can do if the drug is there in your body. To test for this, you can buy a commercial do-at-home test here.
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