I complete tamoxifen therapy (chemoprevention for adh) on 1 jan. should i wait til then to start methotrexate (recently diagnosed with ra)?

Consult physician: Tamoxifen has an interaction with Methotrexate of moderate severity. It causes additive toxicity. Consult your physician regarding the relative necessity of starting Methotrexate now versus risk of toxicity.

Don't have to wait: There is no significant interaction between tamoxifen and methotrexate, so you can start any time. Methotrexate is a disease-modifying agent and takes couple of months to work, thus starting earlier is better, for most cases. It is only a month and a half before completing tamoxifen, so it is not that long of a wait if you so choose. I would advise informing your doc of your concern. Good luck.