Positives

• Azathioprine comes up in ME/CFS and Long COVID as a broad-acting immunosuppressive option for patients with evidence of autoimmune involvement, or who have responded partially to gentler immune-modulating treatments and need something stronger FDA.

Negatives

• It can take 6 to 12 weeks before any benefit appears in autoimmune use, and patients commonly stop because of nausea, abdominal upset, and aphthous ulcers American College of Rheumatology.
• Long-term use is associated with bone marrow suppression and an increased risk of lymphoma and skin cancer, which weighs heavily in patients considering it for off-label Long COVID autoimmune overlap NIH LiverTox.
• Acute pancreatitis and a flu-like hypersensitivity reaction are recognized causes of discontinuation in autoimmune patients PMC.

Hurdles & Side Effects

• TPMT enzyme testing is required before starting — a genetic subset of the population cannot metabolize azathioprine normally and develops severe myelosuppression at standard doses, per the FDA prescribing information FDA.
• Ongoing monitoring includes complete blood count and liver function testing at FDA-label intervals — the drug is not something to "try for a month and see"; the side-effect surveillance schedule is part of the treatment FDA.