How long does cannabis stay in the system
How long drugs are detectable in the system depends on a number of factors:
The drug taken
How much and how often taken
Sensitivity of testing used
Body fluid/tissue used (blood, plasma, urine, saliva, sweat, hair)
Other substances taken (e.g. dilution of urine sample, masking agents or adulterants)
Most urine tests are simple colour-change screening devices, which can react positively to other inactive substances, and should always be confirmed with a detailed GCMS forensic analysis (Gas Chromatography-Mass Spectrometry).
A single hit of cannabis may disappear in a few days, whereas heavy or chronic use can be detected in urine up to a month after stopping. Amphetamine is detectable for up to a week.
A low level of cannabinoids in urine can readily be produced from passive exposure to cannabis smoke, from being in a room, car, bar, concert etc with others who are using. If a responsible test is used, there should be a cut-off level below which a negative result would result, to avoid 'false' positives from passive exposure. The more sensitive the test, the greater the number of false positives, and the longer period for detecting drugs in urine after the last use.
One way to minimise the risk of a positive result is to avoid giving a sample from the first pee of the day, and to maximise the volume of urine produced e.g. by drinking plenty of water or coffee (caffeine is a diuretic) before the test. Several websites offer various products claimed to beat the drug tests, but I am unaware of any which are effective every time. A money back guarantee is no compensation for losing a job, and a legal principle prevents you suing them.
Several websites are listed in our drug testing and forensic links page.