Can You Cheat a Drug Test?

The answer, generally speaking, is no. The Internet is swarming with popular ways to cheat a drug test, ranging from sample switching to sample alteration to making up clever excuses. Although some of these methods can work some of the time, none of them can reliably guarantee a negative test.
Sample Switching
One of the most common ways people try to beat a drug test is by switching their drug-laden urine sample with a clean sample. This may be synthetic or real urine bought off the Internet or provided by a friend. Sample switching is an ineffective method for two reasons. First, most tests involve measures designed to make switching difficult or impossible. Some testing companies ask the person being tested to urinate in front of a witness, while others require changing into a hospital gown to make smuggling items into the testing area difficult. Second, the test itself is sensitive enough to detect an old or synthetic sample, meaning that at the very best, switched samples are flagged for more testing.
Adulteration 
Various theories on cheating drug tests advise adding bleach, Visine, Drano or one of several other chemicals to the sample. Supposedly, this will clean out the THC and cause a negative test. Beyond the challenge of smuggling these chemicals into the testing area, they are ineffective when used. What these chemicals actually do is alter the pH level of the sample far outside normal human levels. Testing companies immediately know when samples have been altered, and will perform additional tests on the suspicious sample. Some people try to alter their urine by drinking vinegar or pickle juice, which is not only unhelpful but also causes violent diarrhea.
Dilution
In order to test positive, a urine sample must have a THC level above a certain threshold. The theory behind dilution is that drinking large amounts of water or juice will add more fluid to the urine, thus reducing the THC concentration. Dilution really does lower the amount of THC in a urine sample, but it also dilutes other chemicals typically found in urine. The testing company knows this and carefully screens each sample for signs of deliberate dilution. A particularly dilute sample may be subject to more intensive testing, or the person who provided the sample may be called in for another test with little time to prepare.
Excuses
Some people try to explain away a positive test by claiming to be passive users. Perhaps they accidentally inhaled some marijuana at a party or rock concert. Others try to get out of the test by eating poppy seed muffins, which contain trace amounts of opiates. Modern drug tests, though, set a threshold for a positive test that is far above what could appear in a casual or accidental user’s urine. Anyone who deliberately uses drugs will test positive, and these excuses will not be seen as valid explanations. Conversely, people who really are passive or accidental users have nothing to worry about, since their THC levels are below the minimum threshold for a positive test.
People have tried all of these methods and more to cheat on drug tests for years, and testing companies know every trick in the proverbial book. They have devised drug tests sensitive and secure enough to beat every one of these methods time and again. Although it is remotely possible to beat the test on occasion, no method of cheating can guarantee a negative test. Anyone who continues to use drugs and continues to be tested will get caught, sooner rather than later. The only one hundred percent effective way to guarantee a negative drug test is to stop using drugs