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Compulsory drug testing was introduced in 1968 by the International Olympic Committee. Since then, several doping cases have been reported in sports competition world wide. Positive results are based on the detection of prohibited substances, their metabolites and markers in biological (mainly urine) samples supplied by athletes. In some cases, the evidences were not contested and athletes admitted the use of banned substances. However, in other cases, athletes denied the use of doping to enhance performance and claimed to have inadvertently or passively absorbed the drug. Unfortunately, no current accepted analytical method is capable of distinguishing between a sample from a cheater and one from an athlete who was passively exposed to a doping agent. Athletes' allegations have included the passive inhalation of drug smoke (e.g. marijuana) or the ingestion of food or products sold as nutritional supplements that contained prohibited substances. In the scientific literature, several studies have been performed to investigate the possibility of an accidental exposure being the reason for the appearance of detectable quantities of banned substances in urine samples. Based on these studies, this article discusses those cases where the athlete's claims could be possible in generating a positive result in doping control and in which circumstances it would be improbable to happen.

Although not yet fully recognised by international sporting committees, hair analysis in doping control may be a useful adjunct to drug testing of urine. It may permit access to retrospective information and the identification of banned substances, especially when exogenous abuse has to be distinguished from other forms of involuntary exposure to identical substances. Negative hair results coupled with positive urine samples may be used to draw conclusions of involuntary doping in sports whenever athletes claim not to have ingested any drug, identical substances are present in their environment or are normal constituents of food and beverages served to them immediately before the competition. Two cases are well described in the literature in which hair analyses were fundamental in documenting positive doping after urinalysis. In Brazil, 2 cases of athletes testing positive for banned substances caught our attention because of the possibility of involuntary doping; hair analysis, if performed, may have helped to clarify the results of the urinalysis. Despite the fact that it cannot be used for routine control and overrule positive urinalysis, hair analysis can detect long term exposure as well as those substances which are not excreted in urine. In the current International Olympic Committee (IOC) code, hair analysis is not yet considered useful even in special cases of doping control.