There are many situations in which you may have to provide a sample of urine, saliva or hair for drug testing. The results can affect your eligibility for a job and possibly result in criminal charges.
However, drug testing is not infallible. According to Drugs.com, there are substances that can result in false positives. These may be perfectly legal and acceptable for you to have in your system.
Sertraline is an antidepressant medication. Sold under the brand name Zoloft, it treats depression and anxiety disorders. However, if you have to take a drug test while taking sertraline, it can result in a false positive for benzodiazepines, one of the substances that drug testing screens for.
Poppy seeds are an ingredient in many pastries and baked goods, added for flavoring. The seeds used in baking do not come from the opium poppy. Nevertheless, it is possible that if you eat something containing poppy seeds, it could produce a false positive for opioids. This is more likely with urine testing than with saliva testing. Cooking the seeds makes them more palatable and also renders them less likely to produce false positives than raw seeds.
Dextromethorphan is an ingredient in over-the-counter cough suppressants, which you might take if you have a cold. There is a chemical relationship between DXM and a narcotic analgesic, so it can produce a false positive for opioids. There are also reports of false-positive results for angel dust, a hallucinogen, due to DXM.
If you have consumed a substance that could produce a false positive, you should inform whoever is conducting the test ahead of time. Any positive results should go through secondary confirmatory testing that should show the positive to be false.