The research described in this thesis is based on novel analytical approaches to develop new methods for determining psychoactive and therapeutic drugs in biological matrices. All the analytical methods developed were according to Food and Drug Administration (FDA) guidelines. The first project involved development and validation of an analytical method for quantification of psychoactive drug mephedrone and its two metabolites 4- methylephedrine and 4-methylnorephedrine in human hair using liquid chromatography tandem mass spectrometry (LC-MS/MS). Recent abuse of designer drugs such as mephedrone has presented a requirement for sensitive, reliable and reproducible analytical methods for the detection of these controlled drugs in different matrices. Based on the similar structure of mephedrone to other methcathinones and amphetamines, the study also proposed a similar metabolic pathway for mephedrone. The method developed can be of great value for the future detection of mephedrone and its two metabolites in human hair. Having completed the above, another analytical method was developed capable of detecting 0.6 ng/mg abacavir and tenofovir in human hair using LC-MS/MS. The method was successfully validated for the intraday precision, interday precision, and limit of detection, accuracy and extraction recovery. This is the first full report of a method for the simultaneous determination of these two key antiretroviral drugs in hair. The newly developed method is useful for future routine analysis of tenofovir and abacavir in human hair and could be used in therapeutic drug monitoring and adherence to medicines studies, which would be helpful in decision making regarding treatment change in combination anti-retroviral therapies. The last project focused on analysing dietary substances such as green and white tea, fruit juices along with catechins present in tea and corticosteroids in order to investigate potential inhibitory effects on the glucuronidation of B2" agonists clenbuterol and formoterol. [3;- agonists are frequently prescribed for the treatment of asthma in athletes. Due to performance-enhancing effects, these [3;- agonists have been subjected to restrictions in sport. A glucuronidation method using human liver microsomes (HLM) and uridine 5'-diphospho-glucuronosyltransferase (UGT2B17) has been used. The study shows that common dietary products and the catechins present in tea inhibit the glucuronidation activity of forrnoterol which effects/alters the actual drug concentration when testing in human body/urine and may help in masking formoterol misuse therefore having implications on current doping control in sport.