Drug testing is helpful (even indispensable) as a way of motivating drug and alcohol dependent people to remain clean and sober. Drug and alcohol dependent people are often in denial about the nature and extent of their problems and typically do not want (wholeheartedly) to quit, at least the first time or two they enter treatment. They often lie about their substance use, either denying that they have used at all, or minimizing the amount of drugs or alcohol that they have used. It is necessary to obtain objective and indisputable evidence of whether or not a drug and alcohol dependent person is using drugs or alcohol during the early and middle phases of treatment.
Most drug tests ('screens') are performed on urine samples that drug dependent persons must provide twice a week (the detection limit for many drugs being about three days in length). In addition to urine sampling, drug testing may occur variously by way of breath tests, or sampling of blood, hair, saliva and sweat. Ideally, urine (or other body substance being sampled) is collected with a clinical worker present in the bathroom (so as to verify that the urine presented in the sample is in fact from the body of the drug dependent person and not from a plastic bag they have carried, and that the urine is not adulterated in any way). Some drug or alcohol dependent people are in such denial that they will maintain that they have not used, even when test results clearly indicate that they have used drugs or alcohol, so it is helpful for clinicians to set an up front treatment policy that drug test results will be believed over the word of drug dependent persons when there is any question of use.