The only way to cleanse your system is to stop smoking until your body flushes the THC out.
Regardless of what you might hear, “home remedies” like niacin, cranberry juice/pills, pickle juice, vinegar, or bleach do NOT work. Moreover, some of these “home remedies” can cause serious, often irreparable damage, and can even kill you. In massive doses, niacin can cause liver damage, or even complete liver failure. Bleach is a poison and will also put you in the hospital or even kill you.
Unlike most other drugs, THC is fat-soluble: it stores itself in your fat cells. That’s why drinking water won’t do anything to remove it from your system, and that’s why it takes such a long time to get it out of your system. Drugs like opiates or cocaine or speed stay in your bloodstream, so you can pee them out in a few days, but since THC stores itself in your fat cells, you basically have to work it off, like, exercise it off, which takes weeks, depending on your height and body weight. If you’re short and skinny and you exercise a lot, you might have to stop smoking for only 2 weeks or so, but if you’re large and you don’t exercise, you’ll have to wait a month, or even six weeks.
Now, if you want to “cleanse” your system more quickly, there are detoxifying drinks you can take. Except, they don’t actually clean the THC from your system. They just mask it so it is undetectable in a drug test. Of course, this is assuming those detox drinks actually work, which may not be a safe assumption to make.
I took one once, called “Herbal Clean.” I passed, but I can’t conclusively credit that to the drink. I also stopped smoking for 9 days prior, and I am small and slender and I work out a lot. But at the time, I had been smoking 3 to 4 times a day for several years, so it seems unlikely that I could have passed naturally. So it may have been the drink, or it may have been the 9-day abstinence, or a combination of the two. Or, it could have been a lab error. I really don’t know.
You can buy these at GNC or other health stores like that. Except, they’re just called detoxifying drinks or something. But if you ask the salespeople, they will usually know what you’re talking about, and will find it for you.
Note that buying these drinks for the purpose of fooling a drug test is illegal. However, buying the drinks for any other reason is not illegal. So, do not say to the salespeople, “I need a drink to pass a drug test.” Tell them, “I want a drink that will detoxify my system of drugs so I can get healthy.” Or better yet, just tell them, “I need some Herbal Clean.” Why you need it is none of their business. As long as you do not tell anyone that you want to pass a drug test, it is perfectly legal.
Depending on how long you have been smoking and how often you do it, you should exercise. Since a lot of people say that THC clings to the fat in your body then my advice would be to exercise regularly and drink plenty of fluids. Especially water and cranberry juice.