Methotrexate (MTX) is a major drug used in the treatment of rheumatoid arthritis (RA). Methotrexate can cause adverse effects such as mucositis, interstitial lung disease (ILD), infections, hepatotoxicity, and myelosuppression, even though it is useful for the treatment of rheumatoid arthritis. One well-known pernicious effect of methotrexate is dose-related mucositis. This study describes a 71-year-old female patient with Rheumatoid Arthritis (RA) who developed methotrexate-induced mucositis as a result of an unintentional overdose. Over the past week, she has developed subtle, progressive mouth ulcers and odynophagia. Early indications of bone marrow suppression were confirmed by her initial laboratory tests. Leucovorin calcium 50 mg was given to reduce the toxic effects of MTX, like bone marrow suppression. Benzydamine mouth wash was given to treat mouth ulcers. The patient, who was hospitalized for four days, was discharged with improved health and healed oral ulcerations. The MTX overdose was caused by taking a daily dose instead of a weekly dose. Thus, it is imperative to provide precise dosage guidelines and emphasize the vital function of folic acid in avoiding MTX toxicity.