That happened to me, and while I paid the first time out of pocket, the second treatment I went through the mfgr’s patient
assistance program; they paid for 100% of both meds, with no
copay or deductible to meet.
Also, these drugs often require prior authorization; sometimes that can take a week or more to arrange. It’s not quite like filling a prescription for penicillin or something; it’s often a bit more involved.