In the U.S., the FDA approves a drug for a particular use. Rituximab would have to undergo a complete set of clinical trials to show its safety and efficacy for

In the U.S., the FDA approves a drug for a particular use. Rituximab would have to undergo a complete set of clinical trials to show its safety and efficacy for MS in particular. Otherwise doctors can only prescribe it off-label, and the system (i.e., liability) is set up to dissuade doctors from prescribing drugs off-label if there exists a drug approved by the FDA for the particular purpose.