I'm going to chime in here - I'm a clinical pharmacist, so your OT caught my eye!
Methotrexate is used for a number of different indications, sometimes at low doses, like for RA, and sometimes at extremely high doses, like for different malignancies. The most serious side effects (which are probably the ones you found that scared you!) are more common with the high doses. When patients take methotrexate for RA, they generally have their liver function, kidney function, and blood counts measured 3-4 times a year, maybe more frequently when the drug is first started. That way, potential toxicity can be detected before it becomes a problem.
Methotrexate for RA is usually taken 1 to 3 days per week; one of the errors I see not infrequently is people mistakenly taking a dose every day. If your mom is prescribed a daily dose, it is probably the total weekly dose divided into 7 days (so a really really small milligram daily dose), but it would be worth double checking that with the doctor and the pharmacist who fills the prescription.
Ask lots of questions so you can be sure you and your mom understand the regimen and the monitoring!
