Methotrexate is one of the most commonly used anti-neoplastic and disease modifying drug today. Side effects of methotrexate at high dose may be life threatening; however, these side effects oral dosing are variable because of the inter-individual variability of gastrointestinal absorption of this drug. Because of confusing nature of its dosing schedule and different routes of administration there are high chances of misinterpretation of prescription. Medication errors involving methotrexate revealed that errors occurred during all phases of use, often resulted from confusion about dosage, and often caused death or other serious adverse effects. Here we present three cases of toxicity due to methotrexate medication error. All cases presented with multiple erosions and ulcers on skin and mucous membrane, organ failures, bone marrow suppression.