Written by Keen Meds · Reviewed by Licensed Clinical Providers · March 1, 2026
Key Insight: The Cannarella 2024 VTE meta-analysis (Asian Journal of Andrology) found no statistically significant increase in venous thromboembolism risk associated with testosterone therapy in men without pre-existing VTE risk factors, though men with hereditary thrombophilia require individualized risk assessment.
One concern that men commonly encounter when researching testosterone therapy is the question of blood clots. FDA prescribing information for testosterone products includes a warning about venous thromboembolism (VTE) -- the category that includes deep vein thrombosis (DVT) and pulmonary embolism (PE). This warning has understandably created apprehension. But prescribing label warnings are written for regulatory caution, not to communicate precise risk magnitude. The actual research on TRT and VTE risk is more nuanced and, for most men, considerably more reassuring. The Cannarella 2024 meta-analysis, published in the Asian Journal of Andrology, provides the most current and comprehensive synthesis of this evidence. This article explains what VTE is, why there was concern about TRT and clotting, what the meta-analysis found, and what clinical monitoring addresses the underlying biological risk.
Understanding Venous Thromboembolism and Why TRT Was a Concern
Venous thromboembolism refers to blood clots that form in the venous system. A deep vein thrombosis is a clot that typically forms in the deep veins of the legs. A pulmonary embolism occurs when a clot breaks free and travels to the lungs, potentially blocking blood flow and causing a life-threatening emergency.
The concern about testosterone therapy and VTE derives primarily from one established pharmacological effect: testosterone stimulates erythropoietin production in the kidneys, which in turn drives increased red blood cell production in the bone marrow. This is a well-understood and expected physiological response. The consequence is a rise in hematocrit -- the proportion of red blood cells in a volume of blood -- a condition called polycythemia or erythrocytosis when elevated.
As hematocrit rises, blood viscosity increases. Thicker blood flows more sluggishly, which in theory increases the likelihood of clot formation, particularly in the slow-moving blood of deep veins. This is the biological mechanism that links TRT to VTE risk -- not a direct clotting effect, but an indirect one mediated through polycythemia.
Given this mechanism, concern about TRT and VTE is scientifically reasonable. The question is: does the theoretical mechanism translate into a clinically significant increase in VTE rates in real men on TRT? That is what the Cannarella 2024 meta-analysis was designed to assess.
What the Cannarella 2024 Meta-Analysis Found
Cannarella and colleagues systematically identified and analyzed randomized controlled trial data examining VTE rates in men receiving testosterone therapy compared to placebo. The meta-analysis was published in the Asian Journal of Andrology in 2024.
The key finding was that across the pooled RCT data, there was no statistically significant increase in VTE rates in men receiving testosterone therapy compared to men receiving placebo. In the general TRT population studied in randomized trials, the theoretical biological mechanism did not translate into a detectable increase in blood clot events.
This finding does not mean VTE risk is zero. Rather, it means the risk increase, if any, is small enough that it was not detectable as a statistically significant signal across the pooled randomized trial data. For most men on appropriate-dose TRT with standard monitoring, VTE risk does not appear to be meaningfully elevated compared to men not on TRT.
The meta-analysis also identified an important stratification: men with pre-existing clotting disorders, specifically hereditary thrombophilia conditions such as Factor V Leiden mutation or antiphospholipid syndrome, represent a different risk profile. These men have an underlying tendency toward clot formation that is independent of TRT, and adding the hematocrit-raising effect of testosterone to this pre-existing risk requires individualized assessment. For these men, the risks and benefits of TRT must be carefully weighed by a clinical provider.
The Hematocrit Connection: Monitoring as the Risk Management Tool
The biological mechanism linking TRT to VTE -- polycythemia leading to increased blood viscosity -- is real even if the resulting VTE risk increase is modest in unselected populations. This is why hematocrit monitoring is not optional in TRT management.
Clinical guidelines from the Endocrine Society and other major professional organizations establish specific hematocrit thresholds for managing TRT. A hematocrit at or above 54% is the standard action threshold at which clinical intervention is required. At this level, blood viscosity increases in ways that may meaningfully raise clotting risk, and continuing TRT without intervention would not be appropriate.
When hematocrit exceeds 54%, clinical options include dose reduction, change in delivery formulation, therapeutic phlebotomy (donating blood, which lowers red blood cell concentration), or temporary treatment pause while hematocrit normalizes. In practice, most men on appropriately dosed daily transdermal testosterone do not develop hematocrit in this range. Polycythemia is more commonly associated with injectable TRT formulations, which produce higher peak testosterone levels due to their pharmacokinetic profile.
The monitoring schedule for hematocrit is standardized: a baseline reading before starting TRT, a follow-up at 3 months (when TRT's erythropoietic effect has stabilized), and then at minimum annually for stable patients. This monitoring protocol is the clinical mechanism that keeps hematocrit within a safe range and prevents the theoretical VTE risk from becoming an actual one.
Risk Factors That Warrant Additional Attention
While the general TRT population does not show a statistically significant VTE elevation in randomized data, certain men should receive more careful individualized risk assessment before starting therapy.
Men with a personal history of DVT or PE have demonstrated VTE risk that is independent of testosterone. Starting TRT in this population requires explicit discussion of the risk-benefit balance, often in consultation with a hematologist or specialist in coagulation.
Men with known hereditary thrombophilia -- inherited genetic mutations that predispose to clotting -- similarly require individualized evaluation. Common hereditary thrombophilias include Factor V Leiden mutation (the most prevalent), prothrombin gene mutation, protein C or protein S deficiency, and antithrombin III deficiency. Men with unknown personal or family history of clotting events should consider thrombophilia screening before starting TRT if there is clinical suspicion.
Men who smoke have an elevated baseline VTE risk, as do men with active malignancy, prolonged immobility, or morbid obesity. These pre-existing risk factors are cumulative with any TRT-related contribution, however modest.
Men with sleep apnea should also be aware that untreated sleep apnea independently raises erythropoiesis, and the combination of sleep apnea-driven and TRT-driven erythrocytosis can push hematocrit higher than either alone would.
Frequently Asked Questions
Does testosterone therapy cause blood clots?
Randomized controlled trial data, synthesized in the Cannarella 2024 meta-analysis, did not show a statistically significant increase in VTE rates in unselected men on TRT compared to placebo. The biological mechanism for theoretical risk -- polycythemia increasing blood viscosity -- is real and is why hematocrit monitoring is standard in TRT management. Men with pre-existing clotting disorders or elevated hematocrit require more careful individual risk assessment.
What is the clinical action threshold for hematocrit on TRT?
The established clinical threshold is 54%. Hematocrit at or above this level is the standard trigger for dose reduction, formulation change, or therapeutic phlebotomy. Most clinical guidelines, including the Endocrine Society and BSSM, use this threshold. Routine monitoring at 3 months and annually thereafter allows early detection and management.
Is polycythemia from TRT common?
Clinically significant polycythemia requiring intervention occurs in approximately 2 to 5% of men on TRT in clinical trial populations. It is more common with injectable formulations that produce higher peak testosterone levels compared to daily transdermal delivery. Transdermal testosterone (spray, gel) typically produces lower and more stable levels, with a lower associated rate of polycythemia.
Should I be tested for clotting disorders before starting testosterone therapy?
Men with a personal history of DVT or PE, or a family history of clotting events, should discuss thrombophilia screening with their provider before starting TRT. For men without such history and without other significant VTE risk factors, routine thrombophilia screening before TRT is not universally required by current guidelines, but the conversation is appropriate if risk factors are present. A clinical provider can help assess whether screening is indicated for your specific situation.
What This Means for Your Treatment Decisions
The Cannarella 2024 meta-analysis provides reassurance that VTE risk in men without pre-existing clotting disorders is not significantly elevated by testosterone therapy in randomized trial data. The biological mechanism for risk is managed through standard hematocrit monitoring -- which is built into responsible TRT clinical practice. For men appropriately selected and monitored, this is not a reason to forgo a clinically beneficial treatment.
Keen Meds connects you with licensed clinical providers for a telehealth evaluation and lab review. If your testosterone is clinically low, you may qualify for Testosterone Spray Rx.
