Methotrexate is a cytotoxic agent that is commonly used to treat autoimmune diseases and both solid and haematological malignancies. Drug interactions are of particular concern during methotrexate therapy, as they have the potential to alter the pharmacokinetics of methotrexate and lead to significant toxicity. One agent that has the potential to interact with methotrexate is the antimicrobial agent trimethoprim/sulfamethoxazole. Consideration of this interaction is of importance in patients that are undergoing high dose methotrexate therapy for haematological malignancies as trimethoprim/sulfamethoxazole is the preferred agent for Pneumocystis Jiroveci pneumonia (PJP) prophylaxis. Current Australian guidelines recommend avoiding concurrent administration of high dose methotrexate and trimethoprim/sulfamethoxazole, yet evidence to support this recommendation is unclear. A review of the literature found eight clinical studies and one case report that investigated the interaction between methotrexate and trimethoprim/sulfamethoxazole in patients with haematological malignancies. Of the eight clinical studies, five either recommended the use of trimethoprim/sulfamethoxazole during high dose methotrexate treatment or found no interaction between the two agents. Furthermore, this review identified a potential link between dose frequency of trimethoprim/sulfamethoxazole and methotrexate toxicity in this patient population. In conclusion, the findings of this review suggest that concurrent use of trimethoprim/sulfamethoxazole, when dosed intermittently, during high dose methotrexate treatment for haematological malignancies appears to be safe and effective in the prevention of PJP.