I know for a fact that in my town, which is an extremely liberal city with several head shops, some employers test for the presence of certain chemicals known to reduce the efficacy of drug tests, like
niacin and bleach. Also, if you think you can drink a lot of water until your pee runs clear, you're wrong because they are very wise to that and will order a re-test, which may be good because it will buy you some time to really detox.
Opiates stay in your system according to how much you have built up in your system from either really heavy use (weeks-months) or once-weekly users (days-a week).
Amphetamines run your metabolism faster and shouldn't show up after four or five days after use, and tetrahydrocannabinol (THC) in
marijuana is stored in fat cells, so if you're a chubbier guy, it will take longer than a 100-lb teenager. Unfortunately, THC has a much longer stay in your body. IT has taken some of my friends who were very heavy users (high constantly all day) up to six months to comletely empty their system.
Good luck, because sometimes if they suspect you have been taking a detox treatment or diluted your urine, they'll order a hair follicle or saliva test which are MUCH more accurate.
There are also kits, if you've ever read a High Times magazine, that include a fake penis and bag of actual clean urine that you can store in your pants before the test until the bag of urine is the right temperature, but that seems like a bit much to me. Good idea, though.
Jesus Christ will help you through. -Betsy