It depends on a couple of factors, but basically they're looking for the highest efficiency medication that they think is safe, your body will tolerate, and that will be covered (or mostly covered) by your insurance. My neurologist laid out my options and actually explained the benefits and any risks. I started on Tysabri, which is one of the strongest DMTs out there, but he explained that for me, he did not advise it as a long term treatment. Many people do just fine on Tysabri. I just had some things specific to me that made it a temporary med.

Tysabri is typically used for active Relapse Remission, which is what most of us are initially diagnosed with. There are a couple of other meds that treat RRMS, but Tysabri and Ocrevus are often the first lines after diagnosis. There are some oral treatments and injectables. They can usually distinguish between RRMS and Clinically Isolated MS or Primary Progressive. Some of us eventually transition to Secondary Progressive MS, but that assessment is made after months or years of increased disability without distincted relapses. Some of the oral meds (Movenclad) have more risk and they like to see how we do on an infusion first. Your neuro should be going over your treatment with you and if there are things you don't understand, you should absolutely ask.