Low-dose azathioprine or mercaptopurine in combination with allopurinol can bypass many adverse drug reactions in patients with inflammatory bowel disease.

Background: The thiopurine drugs, azathioprine and mercaptopurine (MP), are established treatments for IBD. However, therapeutic failure caused by adverse drug reactions occurs frequently.

Aim: To study combination of allopurinol with reduced-dose thiopurine in an attempt to avoid adverse drug reactions in the treatment of IBD.

Methods: Patients with drug reactions to full-dose thiopurines were recruited for combination therapy in two IBD centres in this retrospective study. Dosing was guided by measuring thiopurine methyltransferase (for UK patients) or thioguanine nucleotides and methyl-6MP (Australian patients). Response was monitored by clinical activity indices.

Results: Of 41 patients, 25 had non-hepatic and 16 had hepatitic reactions. Clinical remission was achieved in 32 patients (78%) with a median follow-up of 41 weeks (range 0.5-400). Patients who did not respond to combination therapy tended to fail early with the same adverse reaction. The relative risk of having an adverse reaction with methyl-6MP in the top interquartile range was 2.7 (1.3-28) times that with methyl-6MP in the lower three quartiles (95% confidence interval).

Conclusion: The combined experience from our centres is the largest reported experience of this combination therapy strategy in IBD, and the first to provide evidence for benefit in thiopurine and allopurinol co-therapy to avoid non-hepatitic adverse drug reactions.