If your MS team conclude your treatment is no longer working or you develop serious complications from the drug then they may suggest you stop treatment. Even if the drug has been effective in the past, there may come a time when treatment is no longer effective or you are at too high a risk of the complications to continue.
Alternatively the nature of your MS may have changed. A proportion of people will develop secondary progressive MS meaning they have fewer or no relapses but have a gradual increase in disability. While you continue to have relapses or show evidence of MS activity on MRI scans, it may be appropriate for you to continue taking one of the DMDs. The progressive phase is thought to be caused by permanent loss of nerves rather than new inflammation, so the DMD drugs are not effective or useful in this stage.
Your team should discuss the reasons why treatment may need to be stopped, give advice on an appropriate time to do this and give support throughout the process.