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Leptomeningeal spread of cancer is rare, difficult to both diagnostically confirm and treat, and associated with a poor prognosis. The blood–brain barrier largely prevents sufficient penetration of systemic therapy to be effective. Direct administration of intrathecal therapy has thus been used as an alternative treatment option. We present a case of breast cancer complicated by leptomeningeal spread. Intrathecal methotrexate was initiated, and the manifestation of systemic side effects suggested systemic absorption. This was subsequently confirmed by blood work showing detectable methotrexate levels following intrathecal administration as well as resolution of symptoms with reduction in the dose of methotrexate administered.