What are the adverse effects of methotrexate?

Methotrexate Side Effects

Methotrexate causes common minor toxicities (nausea, stomatitis, fatigue) and three major life-threatening toxicities: myelosuppression, hepatotoxicity, and pulmonary fibrosis, with myelosuppression being the leading cause of methotrexate-related fatalities. 1, 2

Common Minor Side Effects

The most frequently encountered adverse effects include:

• Gastrointestinal: Nausea, anorexia, stomatitis, abdominal distress, vomiting, diarrhea 1, 2
• Constitutional: Malaise, fatigue, chills, fever 2
• Mucocutaneous: Oral ulceration, alopecia 1, 2
• Neurological: Dizziness, headaches, drowsiness 2

These minor effects typically occur at the time of methotrexate administration and can be minimized by administering the drug intramuscularly or subcutaneously, splitting the dose, using folate supplementation (1-5 mg daily except on methotrexate day), or taking the medication with food or at bedtime 1, 3.

Major Life-Threatening Toxicities

Myelosuppression (Most Fatal)

Of 164 methotrexate-associated fatalities, 67 were caused by myelosuppression—making it the single most lethal complication 1. This manifests as:

• Leukopenia, neutropenia, thrombocytopenia, pancytopenia 2, 4
• Can occur even after single doses, particularly in high-risk patients 1
• May develop as late as 4-6 weeks after dose increases 1

Risk factors for hematologic toxicity:

• Advanced age
• Renal impairment (85% of methotrexate is renally excreted)
• Lack of folate supplementation
• Drug interactions (especially trimethoprim-sulfamethoxazole, NSAIDs)
• Hypoalbuminemia
• Methotrexate dosing errors 1, 3

Management: Folinic acid (leucovorin) at 10 mg/m² is the antidote and must be given immediately when myelosuppression is suspected, with repeat doses every 6 hours 1.

Hepatotoxicity

Hepatotoxicity caused 8 of 164 methotrexate-related deaths 1. The risk is notably higher in psoriasis patients compared to rheumatoid arthritis patients 3.

Risk factors for hepatotoxicity:

• Greater than moderate alcohol consumption (>1 drink/day for women, >2 drinks/day for men) 3
• Obesity (BMI ≥30, especially ≥40) 1, 3
• Diabetes mellitus 1, 3
• Hepatitis B or C infection 3
• Concurrent hepatotoxic drugs (statins, retinoids, leflunomide) 1
• Hyperlipidemia and nonalcoholic fatty liver disease 3

Monitoring approach: The 2020 AAD-NPF guidelines 3 recommend noninvasive baseline liver fibrosis assessment (not baseline biopsy) before starting methotrexate. For patients without risk factors, monitor liver function tests every 3-6 months. For those with risk factors, consider annual GI referral or vibration-controlled transient elastography.

Pulmonary Fibrosis

Pulmonary fibrosis caused 30 of 164 methotrexate-related deaths 1. This severe manifestation must be ruled out in any patient presenting with new pulmonary symptoms such as cough. Notably, this complication is much less common in psoriasis patients than in rheumatoid arthritis patients 1.

System-Specific Adverse Effects

Infectious Complications

• Increased risk of opportunistic infections including Pneumocystis carinii pneumonia (most common), cytomegalovirus, herpes zoster, and fatal sepsis 2
• Immunosuppression increases infection risk, though serious infections may be less frequent than with some biologics 5
• Methotrexate should be withheld during acute infections 1

Dermatologic

• Photosensitivity, erythematous rashes, pruritus, urticaria 2
• Severe reactions: Stevens-Johnson syndrome, toxic epidermal necrolysis, skin necrosis 2
• Increased risk of skin cancer (HR 2.05) 4

Renal

• Severe nephropathy or renal failure, though paradoxically, one large trial showed decreased renal adverse events with methotrexate (HR 0.85) 2, 4
• Requires dose reduction when creatinine clearance is impaired 1

Cardiovascular

• Pericarditis, pericardial effusion, thromboembolic events 2

Neurological

• Transient cognitive dysfunction, mood alteration, leukoencephalopathy 2
• Speech impairment, hemiparesis, seizures (rare) 2

Pregnancy and Reproductive Toxicity

Methotrexate is absolutely contraindicated in pregnancy 1, 3. It causes fetal abnormalities at all gestational ages, with the critical teratogenic period being 6-8 weeks of pregnancy 3.

Contraception requirements:

• Women must wait at least 3 months (or 3 ovulatory cycles) after discontinuation before attempting pregnancy 1
• Men should wait at least 3 months after discontinuation 1
• Effective contraception is mandatory during treatment 3

Critical Monitoring Requirements

Initial monitoring (first month): CBC with differential, liver function tests, creatinine every 7-14 days 6

Maintenance monitoring: Every 3-6 months for CBC, liver function tests, and renal function (assuming no abnormalities) 3

More frequent monitoring needed when:

• Increasing doses (check 4-6 days after dosing to avoid transient elevations) 5
• Risk factors present (renal impairment, elderly, drug interactions) 3
• Dehydration or other conditions increasing methotrexate levels 2

Drug Interactions Increasing Toxicity

Critical interactions that elevate methotrexate levels and toxicity risk 1:

• Decreased renal excretion: NSAIDs, probenecid, salicylates, sulfonamides, colchicine, cyclosporine
• Decreased protein binding: Penicillin, minocycline, trimethoprim, sulfonamides, diphenylhydantoin
• Folate pathway interference: Trimethoprim-sulfamethoxazole (particularly dangerous for bone marrow toxicity) 5

Key Clinical Pitfalls

1. Weekly dosing confusion: Mistaken daily use of the weekly dose has led to fatal toxicity—prescriptions should never be written PRN 2
2. Test dose consideration: A 2.5-5 mg test dose with CBC check 5-6 days later is recommended, especially in patients with renal impairment 1
3. Folic acid is protective: Supplementation (1-5 mg daily except methotrexate day) reduces GI and hepatic adverse effects without significantly compromising efficacy 1, 3
4. Elderly patients: Require particularly close monitoring due to diminished hepatic/renal function and decreased folate stores 2