There are a number of factors that can affect whether or not marijuana will show up on a drug test. Firstly, maybe it was 2 puffs yesterday, but when and how often do you smoke? THC is fat soluble, thus it tends to stay in your system for longer than prescription medication, especially if you are overweight. The estimated time until negative ranges anywhere from 3-30 days, so if you are heavier set and smoked 2 weeks ago chances are good that it will show up. Furthermore, you must consider what kind of test you will be taking. Is an immunoassay (cup test with instant results) or will it be sent to a laboratory for gas or liquid chromatographic confirmation? Immunoassays have higher cutoff levels and are thus less capable of detecting low levels of a drug or its metabolites. If it goes to a lab, there's pretty much no way they're going to miss it. In short, if you've only taken two puffs in the last 30 days and you're only going to be taking a cup test, no, you're probably ok. Otherwise, it will likely show up.