Olsalazine and Mercaptopurine: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Olsalazine
Mercaptopurine
How we grade severity & evidence
Severity levels
- Contraindicated: These should generally not be used together.
- Major: Potentially serious — often needs a change or close monitoring.
- Moderate: Can be significant — usually manageable with monitoring.
- Minor: Usually limited clinical impact.
Evidence grades
- Established: Well documented — supported by controlled studies or strong clinical data.
- Probable: Good supporting evidence, though not definitively proven.
- Suspected: Some evidence suggests this interaction, but it is not well established.
- Possible: Limited or conflicting evidence; the interaction may occur.
- Theoretical: Predicted from the drugs' pharmacology; not yet confirmed in people.
Ratings come from the documented interaction literature and are reviewed by a pharmacist. They describe the documented risk of the combination, not what will necessarily happen to you — your dose, timing, and health picture all matter.
What happens
An increased risk of bone marrow suppression, an increased risk of myelosuppression and an increased risk of hepatotoxicity
Interaction Deep Dive
Coadministration of olsalazine, an aminosalicylate derivative, and mercaptopurine may inhibit the thiopurine methyltransferase enzyme (TPMT) and increase the risk of bone marrow suppression (anemia, leukopenia, thrombocytopenia, or any combination) caused by mercaptopurine. Mercaptopurine is hepatotoxic. Hepatotoxicity may be increased when mercaptopurine is coadministered with other products that can cause hepatoxicity, such as olsalazine. There are reports of deaths attributed to hepatic necrosis associated with the administration of mercaptopurine. Hepatic injury can occur with any dosage but seems to occur with greater frequency when the recommended dosage is exceeded. Monitor serum transaminase levels, alkaline phosphatase, and bilirubin levels at weekly intervals when first beginning therapy and at monthly intervals thereafter. Monitor liver tests more frequently in patients who are receiving mercaptopurine with other hepatotoxic drugs or with known pre-existing liver disease. Withhold mercaptopurine at onset of hepatotoxicity1. In a case report, a 16-year old female diagnosed with Crohn's disease given both drugs developed hematopoietic toxicity 2.
Why it happens (mechanism)
Inhibition of thiopurine methyltransferase (TPMT) by olsalazine; additive myelosuppression and hepatotoxic effects
Literature reports
1 report — tap to read
a) A 16-year old white female diagnosed with Crohn's disease was started on 6-mercaptopurine 50 mg daily, olsalazine 500 mg daily, and predniSONE 5 mg daily. At this time, her WBC count was 8.8 X 10(9)/L. One month later, when olsalazine and 6-mercaptopurine were increased to 1000 mg and 75 mg daily, respectively, her WBC count had decreased to 4.9 X 10(9)/L. Another month later, she developed leukopenia with a WBC count of 1.7 X 10(9)/L. Her 6-mercaptopurine dose was reduced to 50 mg daily, and two weeks later her WBC remained the same. The 6-mercaptopurine dose was then decreased to 25 mg daily, and her WBC count stabilized at 5.2 X 10(9)/L. Because of the continuing exacerbations of her disease, her drug regimen was changed to olsalazine 1750 mg daily, 6-mercaptopurine 50 mg daily, and predniSONE 50 mg daily. Approximately one month later, her WBC count had fallen to 1.3 X 10(9)/L, and the 6-mercaptopurine dose was discontinued. Her WBC recovered to 4.4 X 10(9)/L in one month, and due to the difficulty in controlling the Crohn's disease with steroids alone, 6-mercaptopurine was again initiated at 12.5 mg daily. While taking this reduced dose, she experienced no further hematopoietic toxicity and her disease was controlled 2.
Common questions
Can I take Olsalazine and Mercaptopurine together?
An increased risk of bone marrow suppression, an increased risk of myelosuppression and an increased risk of hepatotoxicity Always confirm with your pharmacist or prescriber before making any change.
How serious is the Olsalazine and Mercaptopurine interaction?
It is rated major. Potentially serious — often needs a change or close monitoring.
How quickly could this interaction happen?
The documented onset is "delayed". Effects tend to build up gradually over days to weeks.
How strong is the evidence for this interaction?
The evidence is graded "probable". Good supporting evidence, though not definitively proven.
Questions for your pharmacist
- Does my dose of Olsalazine or Mercaptopurine need adjusting while I take them together?
- What symptoms should prompt me to call you or my prescriber right away?
- Does the timing of my doses matter for this combination?
- Is there a safer alternative to one of these medications for me?
References (2)
- Product Information: PURIXAN(R) oral suspension, mercaptopurine oral suspension. Rare Disease Therapeutics Inc (per FDA), Franklin, TN, 2024. DailyMed
- Lewis LD, Benin A, Szumlanski CL, et al: Olsalazine and 6-mercaptopurine-related bone marrow suppression: a possible drug-drug interaction. Clin Pharmacol Ther 1997; 62:464-475. PubMed
Keep reading about Olsalazine
Keep reading about Mercaptopurine
These medications also interact with supplements
Prescription drugs aren't the whole picture — herbal and dietary supplements can interact with them too. From the evidence-graded Natural Medicines database:
major · moderate · minor — check everything you take with our drug–supplement interaction checker.
Check another combination
Our instant two-drug interaction checker is almost here.
Still have questions about this combination?
Every question gets a real answer from a licensed pharmacist — free, and usually within a day.
Ask the pharmacist