Several companies normally test for just marijuana, cocaine, opiates, pcp and amphetamines. If you have done something out of this list, you can indeed yell like a genius and feel a lot better. You will probably be okay passing the drug test.
When reality is kicking in and you see that your employer is checking for lot of other stuff, it can be painful. You hope for the best handing over the sample. You sense they are doing lot of things out there. You already given it and you can do nothing of fair marketing or flush thing to appear sober.
To avoid shocks and unnecessary trips, if you are guilty of prescription drugs you rather be honest or get a reschedule. Some employers are interested in honesty and they tend to consider you if you are true about sleep pills in a while. However, even if they hire they are going to put you on follow up tests. If you are desperate to get the job, you can offer to test randomly in the future to prove you are sober yet.
You can effectively handle a situation where you show up with a drug positive. However, you should report the issue before you caught than after. This might work only if you have exceptional skills and proven track of better work in the past. If you mean a potential to stop or if you are negligibly light in what you do, the employer might probably get interested in your honesty. This is just an exceptional event and you cannot count on it as often.
This is worth doing if you are applying for a long-term job or it can also work in short-term contract jobs. Sign up with an agency and they might do what it takes to put you on job despite your positive drug sometimes; however, you will get less pay in hand.