This invention relates to methods and compositions for determining single nucleotide polymorphisms (SNPs) in P450 genes. In preferred embodiments, self extension of interrogation probes is prevented by using novel non self-extension probes and/or methods, thereby improving the specificity and efficiency of P450 SNP detection in target samples with minimal false positive results. The invention thus describes a variety of methods to decrease self-extension of interrogation probes. In addition, this invention provides a unique collection of P450 SNP probes on one assay, primer sequences for specific amplification of each of the seven P450 genes and amplicon control probes to evaluate whether the intended p450 gene targets were amplified successfully. The invention also describes a variety of array platforms for performing the assays of the invention; for example: CodeLink, eSensor, multiplex arrays with cartridges etc., all described herein.