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New legislation aimed at combating driving under the influence of drugs (DUID) in Sweden stipulated zero-concentrations in blood for scheduled substances. DUID suspects (n = 300), with amphetamine as the only psychoactive drug identified in blood, were investigated in relation to age and gender. In a smaller retrospective sample (n = 70) the relationship between clinical tests of impairment were compared with the concentration of amphetamine in blood.All forensic blood samples were subjected to a broad toxicological screening analysis by immunoassay methods [enzyme multiplied immunoassay technique/cloned enzyme donor immunoassay (EMIT/CEDIA)] and positive results were verified by gas chromatography-mass spectrometry (GC-MS). The limit of quantitation (LOQ) for determination of amphetamine in blood was 0.03 mg/l. People suspected of being under the influence of drugs were examined by a physician who asked various questions about state of health and use of drugs and also administered simple psychomotor and cognitive tests of impairment. After conducting these tests the physician concluded whether the suspects were not impaired, slightly, moderately or highly impaired by drugs other than alcohol.Among 300 DUID suspects with amphetamine in blood there were 246 men (82%) and 54 women (18%). Mean age (± SD) of the men was 37.1 ± 8.7 years compared with 35.5 ± 7.1 years for the women (P > 0.05). The frequency distribution of blood amphetamine concentration was positively skewed with mean, median and highest values of 1.0 mg/l, 0.9 mg/l and 7.1 mg/l, respectively. The mean concentrations were slightly higher in the women 1.11 mg/l (median 1.0 mg/l) compared with 0.97 mg/l (median 0.8 mg/l) in the men (P > 0.05). There was a weak but statistically significant correlation between the person's age and the concentration of amphetamine in blood (r= 0.18, P < 0.05). The results of clinical tests of impairment showed no relationship with the concentration of amphetamine in blood according to analysis of variance (P > 0.05).The lack of association between degree of drug influence and the concentration of amphetamine in blood speaks against the notion of introducing concentration per se limits or graded penalties depending on the blood-concentration of this stimulant. Zero-concentration limits or LOQ-limits are a much more pragmatic way to enforce DUID legislation.