There are many prescription drugs that can affect the results of a drug test. This is why a drug testing site will ask you for information about the drugs you may be taking and the amounts at which you are taking them. To avoid a false positive, the lab drug testing technician needs to take into account the types of drugs you may have in your system for medical reasons and rule them out of the overall result. You may be surprised by the impact these drugs can have on your drug test outcome as many of these drugs have similar effects to illegal drugs and several prescribed drugs can cause a result that indicates you have THC in your system.