Procedures (methods): Participants will be admitted for allogeneic HSCT and on the day after transplant randomized to seven days of high dose IV thiamine or placebo. Thiamine levels will be measured weekly and participants will be assessed for evidence of delirium using validated measures. Validated measures will also be used to assess cognitive function, depression, post-traumatic stress symptoms, functional status, and HRQOL prior to hospitalization and at one, three, and six months after transplant.