One of the most difficult parts in forensic toxicology is to interpret obtained drug concentrations. Was it therapeutic, toxic or even lethal to the particular individual that the blood sample was drawn from? Concentrations of opioid drugs are especially difficult to interpret, because of large interindividual differences in innate and acquired tolerance. Tramadol is a complex drug. Not only is it an opioid, it is also a racemic drug with the (+)- and (-)-enantiomers of the parent compound and metabolites showing different pharmacological effects. Further, it is metabolized by polymorphic enzymes, which may affect the amounts of metabolites formed and possibly the enantiomer ratios of the parent compound and its metabolites. It has been speculated that particularly the (+)/(-)-enantiomer ratio of O-desmethyltramadol is related to the risk of adverse effects, and it has been shown that the ratio is affected by CYP2D6 genotype. The overall aim of the thesis was to evaluate if forensic interpretations of tramadol, regarding toxicity and time since drug administration, may be improved by the use of genotyping and enantioselective concentration determination of tramadol and its three main metabolites. To simultaneously quantify the enantiomer concentrations of tramadol, Odesmethyltramadol, N-desmethyltramadol and N,O-didesmethyltramadol in whole blood, a liquid chromatography tandem mass spectrometry (LCMS/MS) method was developed and validated. Genetic variation in CYP2D6, CYP2B6, CYP3A4 (encoding the tramadol metabolizing enzymes), ABCB1 (encoding a transport protein) and OPRM1 (encoding the ?-opioid receptor) was investigated, using pyrosequencing, xTAG, and TaqMan analysis. The methods were applied to the blood samples of two study populations; 19 healthy volunteers administered a therapeutic, single tramadol dose, and 159 tramadol positive autopsy cases. The most important finding was the positive correlations between all four enantiomer ratios and time since tramadol administration in the healthy volunteers. All enantiomer ratios except the one of tramadol was also affected by the CYP2D6 genotype, which was apparent among the autopsy cases as well. Genetic variation in CYP2D6 and possibly CYP2B6 was shown to have an impact on tramadol pharmacokinetics, although no association to neither drug related symptoms nor tramadol related causes of death was found. Tramadol intoxications were predominantly characterized by low age (median 26 years) and male sex, often with a history of substance abuse and with other drugs (at fairly low concentrations) detected in blood. In conclusion, enantiomer concentration determination combined with genotyping seems promising regarding estimations of time since drug administration, although is of low value concerning interpretations of toxicity in autopsy cases.