The combination of thiopurine and methotrexate (MTX) is a standard co-therapy regimen for acute lymphoblastic leukemia (ALL). Despite its efficacy, this regimen is constrained by a narrow therapeutic window and considerable inter-individual variability, which heightens the risk of drug-induced liver injury (DILI). MTX-induced metabolic strain further destabilizes cytokine-sensitive thiopurine detoxification pathways during systemic inflammation. Conventional pharmacogenetic (PGx) testing for TPMT and NUDT15 variants is effective in predicting myelosuppression, but often fails to detect hepatotoxicity as an adverse effect, suggesting a clinically significant genotype-phenotype difference. This review examines the molecular determinants of DILI, emphasizing the role of secondary metabolic pathways and transporter dynamics as key modulators of risk. The study describes cytokine-mediated (IL-6, TNF-α) transcriptional suppression of cytochrome P450 enzymes and hepatic transporters (SLCO1B1, ABCC2/4) not merely as secondary modulators, but as the primary determinants of localized, tissue-specific drug exposure through disrupted nuclear receptor signaling (PXR, CAR, HNF4α). This mechanism promotes functional phenoconversion and toxic molecular shunting, leading to increased intrahepatic drug exposure. It synthesizes the current knowledge on the metabolism of thiopurine and MTX, focusing on the genetic and non-genetic factors influencing toxicity and their interactions. The review also critically evaluates the limitations of static PGx-guided dosing. It highlights the need for comprehensive, real-time risk assessment that integrates gene-environment interactions, multi-omics data, and clinical monitoring to improve precision therapy for ALL. This approach combines extended PGx profiling, transcriptomic monitoring, and clinical biomarker assessment to provide a transformative strategy for precision drug delivery.