Feb 20, 2019
Jeannie asked
My 4-year-old is on oral methotrexate. I understand you need to wait 12 hours after methotrexate before starting antibiotics. Her antibiotics are for 10 days (ear infection). Should I wait 12 hours after the last dose of antibiotics before starting her back on the methotrexate?
At a glance
- Penicillin and related antibiotics (e.g. amoxicillin) can increase methotrexate concentrations by decreasing elimination from the kidneys. This increases the risk of side effects. Nevertheless, they are sometimes used together in certain situations.
Answer
You are correct that there is a significant interaction between some antibiotics and methotrexate. However, not all antibiotics share this interaction.
The most well-known is with penicillin (and its derivatives).
Penicillins reduce the elimination of methotrexate in the kidney, increasing concentrations which in turn could increase the risk of methotrexate-related side effects.
The prescribing information for methotrexate states the following regarding the interaction:
References
- Elsevier ClinicalKey: Penicillin (Accessed 2/19/19)
- Elsevier ClinicalKey: Methotrexate (Accessed 2/19/19)
- Methotrexate Prescribing Information
