We were told that taking B12 supplements every 12-24 hrs may help ease nausea as well as taking folic acid 5mg the day after MTX. How effective is this is managing side effects and intolerance?

There is no evidence to recommend vitamin B12 supplementation to help ease nausea or
other side effects of methotrexate.

Pediatric rheumatologists usually prescribe folic acid or folinic acid (leucovorin)
supplementation, which helps reduce some of the methotrexate side effects, such as
stomach upset, nausea and liver toxicity. There are many ways of prescribing folic acid or
folinic acid (leucovorin). Some patients take it every day whereas others take it once a
week. Your pediatric rheumatologist will discuss the best regime for you.