If methotrexate is working for ctd, d es one still need to take a break from it after a while (does it build up in the body eventually cause problems?

Not necessarily: Methotrexate can cause cumulative toxicity but your physician should be following lab work regularly. While this not a guarantee against toxicity, it is a very good safeguard. Sometimes, if the disease is clearly in remission, it is reasonable to try and reduce the dosage of medication. However, the old adage "if it aint broke, don't fix it"often applies.