In one study, 11% of transmen who took testosterone developed a condition called erythrocytosis, which impacts red blood cells and slows the blood flow.
A study  of 776 transmen who took testosterone demonstrated that 11% of them developed erythrocytosis, a condition which slows the blood flow, and can lead to headaches, confusion, high blood pressure, nosebleeds, blurred vision, itching and fatigue.
 Madsen, M.C., van Dijk, D. Wiepjes, C.M., Conemans, E.B., Thijs, A. & den Heijer, M. (2021). Erythrocytosis in a Large Cohort of Trans Men Using Testosterone: A Long-Term Follow-Up Study on Prevalence, Determinants, and Exposure Years. The Journal of Clinical Endocrinology & Metabolism 106 (6): 1710–1717. [Link]