If i took one small puff of weed can i fail a drugtest?

It depends. A single puff would clear the body in most adults within days. A chronic recurrent user may test positive a month or more after stopping.

If that is the sum total of your exposure, you are likely to pass the test, however, if you were also exposed to second hand smoke from your association with weed users, you could fail the test. Marijuana is a psychoactive drug and will mess with your brain and damage your lungs among other injury. MJ users are called stoners for good reason. Quit using it while you are ahead. Wish you good health