Summarize:

There are many different types of drug tests, so be sure you know which specific tests you will be taking.  Unfortunately, drug tests are not 100-percent accurate and can produce a "false positive" for numerous reasons. If you are an athlete, you may also be tested for substances that are not necessarily illegal, but that have been banned from athletic competition. Ibuprofen (Advil, nuprin) used to produce a false positive for the narcotic marijuana, but this problem has been remedied and is no longer an issue.  Yet, there still remain a number of foods, medications, vitamins and supplements that will produce a false positive.  A teaspoon of poppy seeds is enough to produce a false-positive for opioids.  This is less than one-poppy seed bagel. Many over-the-counter work-out supplements are banned from athletic competition.  If you are an athlete, discuss your supplement options with your coach or trainer. Find out how the test will be administered, and more specifically, try to find out what foods, vitamins, medications, and supplements have been known to trigger a false positive.  The more you know about the specific tests you will be taking, the more you can actively defend yourself against a false positive.  Prescription antidepressants, such as Zoloft, can also produce a false positive.  Athletes should familiarize themselves with substances that have been banned from competition. A guide to substances that have been banned from competition can be found here. This can mean skipping your morning poppy-seed bagel, or opting to pass on any over-the-counter medications that can affect your test.  But, it is a lot easier to prevent a false positive than it is to contest. Athletes should know that certain substances have been banned for use while competing, but are actually allowed during training. When you go in for your drug test, you will be asked to complete a list of all the foods, vitamins, medications, or other supplements that you have been taking.  This will help the screening facility determine a false-positive from an actual positive test, and could be the difference between losing and keeping your job. Athletes who test positive for a banned substance--regardless of its legality--may face punitive action. Increasing your fluid intake will help flush any of the substances that may produce a false positive (such as poppy-seed), so be sure to increase your fluid intake at least 24-hours before your scheduled examination. You want to give the cleanest urine you can to avoid any false positives, so make sure that you do not provide your first urine of the day as a sample. Drink as much water as you can an hour-or-two before your test.
Learn what tests you will be taking. Be aware that certain over the counter medications and vitamins--even foods--can trigger a false positive. Research the limitations of the test(s) you will be taking. Avoid the "substances" that produce a false positive. Keep track of everything you consume. Drink lots of water. Give a clean urine sample.