Drug testing is typically done by using highly sensitive assays that can detect drugs, or their metabolic byproducts in the urine. Typical drugs that are tested for include THC (drug found in marijuana), cocaine, heroine and other opioids, and benzodiazepines (commonly prescribed sedative and anxiety
medication). These tests are quite sensitive, but the actual detectability of the drugs also has to do with how long the drugs and their byproducts stay in the system. For example, THC can often be detected 2-3 weeks after the last use, while a single use of cocaine is often out of the system withing days. Physicians do these test often in order to better understand, or rule out, causes of symptoms you are having. Sometimes these tests are done by places of work that have strict rules about employees and use of illegal drugs.
If you are asking this question because you have been involved in some illicit drug use recently, then I suggest that you schedule an appointment with your primary care physician
. He or she can discuss this use and recommend treatments that can help you quite if needed. Sometimes the best treatment is group discussion similar to alcoholic's anonymous. Good luck.