Minimising Adverse Drug Reactions and Verifying Economic Legitimacy in Children (MARVEL-PIC)

Inclusion Criteria: * Age \< 18 years * New cancer diagnosis or patient receiving HSCT or patient has a relapsed cancer diagnosis and is starting treatment after more than 6 months without. * Starting treatment with a chemotherapeutic agent that is not single agent oral targeted therapy. * Must also be taking a medication for which there is an established CPIC guideline available. * Parent or patient is able and willing to give consent for patient to take part and be followed up for at least 12 weeks. * Patient is amenable to venepuncture and blood draw (5mL ideally with an absolute minimum requirement of 2.5 mL) or has Whole Genome Sequencing available (WGS). * Patient and/or parent is able and willing to sign an informed consent form. * Patient and/or parent is able to complete Ped-PRO-CTCAE survey in English, Italian or Chinese. * Study enrolment limit has not been reached. Exclusion Criteria: * Age \> 18 years. * Patient has a life expectancy estimated to be less than three months by the treating clinical team. * Duration of the drug of inclusion total treatment length is planned to be less than one week. * Patient and/or parent is unable to consent to the study. * Patient and/or parent is unwilling to take part in the study. * Patient and/or parent is able unable to complete Ped-PRO-CTCAE survey in English, Italian or Chinese. * Patient has existing impaired hepatic or renal function for which a lower dose or alternate drug selection are already part of current routine care. * Patient has a glomerular filtration rate of less than 15 mL/min per 1.73m2. * Patient has advanced liver failure.