The length of time required to "clear" an opiate from your system depends on how long you've been on them & how much you've been taking, along with the efficiency of your liver and your kidneys....so, if your drug testing for a job, or more importantly any legal sort of reason (court ordered, etc)...I wouldn't rely on trying to "clear your system" and then saying you don't take a med you've been taking...and you don't have to take all of your vicodin(or whatever) within a prescribed period of time to be current. Once it's prescribed, it's prescribed P.R.N (look at your bottle), literally translates "as needed" for pain....if four months from now, you have pain, and you still have your Vicodin around, you're perfectly safe in taking it and listing it on a drug test as a medication you're currently taking...also, drinking alot of water prior to a drug test alters your urine's specific gravity....which basically measures the amount of solutes vs water in your urine....all drug testing facilities have a cut off(varies from facility to facility)if your specific gravity is below that level, you have to take the test again,it's also a huge RED FLAG, so it's not a good strategy. RELAX, you have legitimate medication, prescribed to you P.R.N....if you needed it, it's ok list it and trust me, they won't ask for the bottle