One of its less known roles is its influence on the production of red blood cells. Polycythemia, a condition characterized by an increased number of red blood cells (RBCs) in the bloodstream, emerges as a notable concern in the context of Testosterone Replacement Therapy (TRT). This increased viscosity can lead to elevated blood pressure and, in some scenarios, a heightened risk of serious health events such as strokes and heart attacks. Polycythemia, in the context of TRT, refers to an increase in the number of red blood cells (RBCs) in the bloodstream. We hypothesized that short-acting testosterone therapy will be physiologic and have lesser effect on hematocrit compared to long-acting TRT. Red blood cells are the part of blood that carry oxygen from your lungs to the rest of your body. They will know what to ask their doctors, what warning signs to watch for, and what steps can be taken to reduce risk. We will start with the basics of what polycythemia is, why it happens with testosterone use, and how common it is. Others may develop it because they live at high altitudes, smoke, or have conditions that lower oxygen levels in the blood. Polycythemia caused by testosterone is different from other types of polycythemia. The goal is not to scare patients, but to give them the information they need to stay safe while benefiting from treatment. Because polycythemia can develop silently, without clear symptoms, routine blood testing is essential. That is why many doctors prefer topical methods for men who already have risk factors for blood problems. Over time, this can push hematocrit and hemoglobin above normal levels. Overall, the formulation and schedule of testosterone therapy strongly influence how much the bone marrow is stimulated. Understanding why this happens helps patients know why regular monitoring is so important. Routine monitoring gives doctors the chance to step in early. The biggest danger with polycythemia is that it often has no symptoms in the early stages. If any of these are above normal, especially hematocrit, the doctor may diagnose polycythemia. If hemoglobin climbs above the upper limit, it supports the diagnosis of polycythemia. That is why most doctors will repeat blood work 3 to 6 months after starting TRT, then again every 6 to 12 months if everything looks stable. Recognizing the symptoms of polycythemia early is important for safety. Understanding why testosterone therapy can cause polycythemia explains why doctors insist on regular blood tests. These combined effects mean that men on testosterone therapy often see their red blood cell counts rise within a few months of starting treatment. Because of these risks, polycythemia is something doctors watch carefully, especially in men receiving testosterone therapy. Because of these risks, polycythemia is one of the most important side effects doctors monitor in men on testosterone therapy. This condition happens when the body makes too many red blood cells. Testosterone stimulates erythropoiesis—the body’s process of creating red blood cells—by increasing the production of erythropoietin, a hormone made by the kidneys. Elevated red blood cell counts may improve oxygen delivery, but when hematocrit rises too high, blood viscosity increases, raising the risk of headaches, fatigue, clotting events, and cardiovascular complication (El-Khatib et al., 2022; Bhasin & Snyder, 2025). Understanding these risk factors is important because it helps doctors and patients watch more closely for early warning signs. Adjusting the dose, changing the type of testosterone, using phlebotomy when needed, and improving lifestyle all help lower the risk of blood thickening. Some may never develop high red blood cell counts, while others may have problems even on low doses. While these options may seem less convenient to some patients, they often provide safer long-term control and lower the risk of blood thickening. Polycythemia is one of the most important side effects that doctors watch for in men using testosterone replacement therapy (TRT). The information should not be considered complete and should not be used in place of a visit, phone or telemedicine call, consultation or advice of your physician or other healthcare provider. Discuss this information with your own physician or healthcare provider to determine what is right for you. While this can be beneficial for people with anemia or low energy, excessive RBC production can become a concern if levels rise too high.
