How frequently do you recommend ordering labs to monitor for side effects of disease-modifying therapies for multiple sclerosis?

Yes, the monitoring depends on which DMT the patient is on.

For patients on B-cell-depleting therapies, I screen for Hep B and latent TB prior to therapy and check a CBC and CMP every 6 months, with yearly immunoglobulin monitoring.

For JCV Ab-negative patients on Tysabri, I check a CBC, CMP, and JCV Ab every 6 months.

For patients on most oral medications including S1p inhibitors and fumarates, I check a CBC and CMP every 6 months. 

These frequencies should be tailored to individual patients if any abnormalities in the labs arise.