This is a randomized double-blind controlled trial in participants undergoing allogeneic HSCT to determine if high dose IV thiamine can prevent delirium and minimize the deleterious impact of delirium on health-related quality of life (HRQOL), functional status, and other neuropsychiatric outcomes. The investigators will recruit 60 patients admitted for allogeneic HSCT at UNC, randomize them to treatment with high dose IV thiamine (n = 30) versus placebo (n = 30), and systematically evaluate all participants for delirium and related comorbidities. The investigators will use the Delirium Rating Scale (DRS) to measure the severity and duration of delirium immediately prior to transplant and after HSCT until 30 days post-transplant or discharge. If delirium is identified, the DRS will be administered daily until delirium resolves. The investigators will obtain thiamine levels and other laboratory parameters associated with delirium the day after transplant, and continue to monitor thiamine levels weekly thereafter. The investigators will also monitor HRQOL, functional status, depression, post-traumatic stress symptoms, and cognitive function prior to transplant and at one, three, and six months after transplant to elucidate the persistent impact of delirium in this population and the potential for thiamine to mitigate these negative outcomes.