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Background: Ocrelizumab is a humanized antiCD20, thought to be a highly effective disease-modifying therapy (DMT). Its most frequent adverse effects are infusion-related reactions (IRRs). To reduce these reactions, the first dose of ocrelizumab is administered as two 300 mg infusions separated by two weeks. However, in the phase II trial of ocrelizumab, severe IRRs were not significantly different between two doses of 600 mg dose (two separate 300 mg doses) and 2000 mg dose (two separate 1000 mg doses). We compared the IRRs in undivided full (one 600 mg) and divided (two 300 mg) doses of ocrelizumab which is the standard protocol., Methods: MS patients (relapsing or primary progressive MS) who are selected to receive ocrelizumab by neurologist or MS fellowship were enrolled in an open-label randomized controlled trial. Iranian biosimilar of the drug (Xacrel ® by Cinnagen, approved by the Iranian Food and Drug Administration in 2021) was used. The participants received the first dose of ocrelizumab as either one 600 mg dose in one session or two 300 mg doses in two weeks apart. IRRs during or in the first 24 h after infusion were recorded., Results: Of 332 participants, 150 received two 300 mg doses, and 182 received one 600 mg dose (by random selection). Life-threatening adverse effects were not observed in both groups. Overnight admission or permanent drug discontinuation was not needed. Temporary drug discontinuation was significantly higher in the one 600 mg dose group (p-value < 0.001). During the infusions, malaise (p-value: 0.003), skin reactions (p-value: 0.04), throat swelling (p-value: 0.03), and dyspnea (p-value: 0.01) were significantly increased in the intervention group. However, in the first 24 h, there was no significant difference between two different treatment protocols (one 600 mg dose or two 300 mg doses) in the onset of IRRS (p-value: 0.12)., Conclusion: These findings suggest one 600 mg dose of ocrelizumab administration for the first dose is relatively safe. With some protocol modifications, it could lead to fewer patient referrals, saving time and cost and improvement the access for patients., (© 2023. The Author(s) under exclusive licence to Belgian Neurological Society.)