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.
Alcohol And Substance Abuse Drug Screening
Drug testing is an invasion of privacy and a human rights violation.
editor's comment: It certainly is an invasion of privacy. It is justified only when someone cannot stop themselves from using and needs external limits to be set on them for their own benefit, or for the benefit of society (when a drug user's out of control behavior might reasonably be linked to future societal harm). My two cents anyway