Methotrexate Safety

Methotrexate is a folate antagonist. By inhibiting dihydrofolate reductase, it reduces the folate available for DNA synthesis and cell replication. At low doses, this effect is used to control inflammatory disease; at high doses, it is used as cytotoxic chemotherapy. The drug does not affect only abnormal cells. Bone marrow, gastrointestinal mucosa, hair follicles, and embryonic tissue also contain rapidly dividing cells, so toxicity may appear as cytopenias, mouth sores, gastrointestinal symptoms, or fetal harm. Hepatic injury and pulmonary toxicity are additional concerns. Renal function is a dosing variable, not merely a laboratory result. Methotrexate is cleared largely through the kidneys. When the eGFR falls, the drug can remain in the body longer, increasing exposure and the risk of marrow suppression, mucositis, liver injury, and delayed high-dose clearance. Dehydration, vomiting, diarrhoea, or reduced urine output can make this risk change quickly. An experienced nurse looks for a pattern rather than waiting for one dramatic symptom. New oral ulcers accompanied by a falling CBC, rising creatinine, or abnormal liver enzymes suggest systemic toxicity rather than an isolated minor adverse effect.