Can ms cause your teeth to become brittle & break off also very prone to decay?

Not directly: There is no direct correlation of ms and dental decay. However, certain medications that are prescribed for ms patients may reduce salivary flow which in turn may result in a greater tendency to get cavities and other oral problems. If you have a lot of cavities, it is more likely related to your diet and poor oral hygiene. Discuss this issue with your dentist.

Not directly: But if you are having coordination problems and can't practice oral hygiene, then you are just as susceptible as someone who chooses not to brush their teeth. Some medications can cause dry mouth which can lead to more dental decay.

Not result of MS: Would have your dentist investigate your dental status, and list of medications, as some meds do cause both jaw and dental problems, but would also investigate vitamin D, Calcium, and Magnesium levels and check for body wide osteopenia.