The heart question: what TRAVERSE found
For years the worry was heart attacks and strokes. TRAVERSE, published in the New England Journal of Medicine in 2023, was built to answer it. Lincoff and colleagues enrolled 5,246 men aged 45 to 80 who already had heart disease or high heart risk, symptoms of low testosterone, and two fasting results below 300 ng/dL (about 10.4 nmol/L).
Half used a daily 1.62% testosterone gel, adjusted to reach 350 to 750 ng/dL (about 12.1 to 26.0 nmol/L). Half used a placebo gel. Over a mean of 33 months, heart attacks, strokes, and cardiovascular deaths occurred in 7.0% of the testosterone group and 7.3% of the placebo group. Testosterone met the trial's test for being no worse than placebo.
After TRAVERSE, the FDA added its results to every testosterone label in 2025 and removed the boxed-warning language about increased cardiovascular risk.
The limits matter. This was a gel, at a moderate target level, over about three years. It does not tell you what higher levels, injections, or decades of use do.
What TRAVERSE saw more of on testosterone
The trial did find side effects. These were more common on testosterone than on placebo:
- Heart rhythm problems needing treatment: 5.2% vs 3.3%
- Atrial fibrillation: 3.5% vs 2.4%
- Acute kidney injury: 2.3% vs 1.5%
- Pulmonary embolism (a blood clot in the lungs): 0.9% vs 0.5%
- Clinical fractures, in a 2024 follow-up analysis: 3.50% vs 2.46%
Blood pressure
The FDA's 2025 label update says ambulatory blood pressure studies confirmed an increase in blood pressure with all testosterone products, class-wide. The FDA added a blood pressure warning to the labels that did not already have one.
MedlinePlus notes that higher blood pressure can raise the risk of heart attack or stroke, and that your doctor may check it before and during treatment. If you already run high, that check matters more.
Red blood cells and hematocrit
Testosterone pushes the body to make more red blood cells. A 2017 review in the Canadian Medical Association Journal called the resulting erythrocytosis (too many red blood cells) a predictable yet underrecognized effect of testosterone. Injected forms raise hematocrit, the share of your blood made of red cells, more than gels.
Thick blood can cause headache, fatigue, blurred vision, and tingling, and it raises the risk of clots. The review recommends checking hematocrit before starting and again at 3, 6, and 12 months. The American guideline advises stopping testosterone if hematocrit goes above 54%; the European guideline suggests considering a blood draw (phlebotomy) at that level.
A single number is not a diagnosis. Your prescriber reads hematocrit alongside your symptoms and dose.
Fertility
Testosterone therapy lowers sperm production, especially at high doses, per MedlinePlus. The Endocrine Society recommends against starting testosterone in men planning to have children in the near term.
If children are still possible for you, raise it before the first prescription, not after.
Prostate and urinary symptoms
In the TRAVERSE prostate analysis by Bhasin and colleagues (2023), about 5,200 men were followed for an average of 33 months. Prostate cancer was diagnosed in under 1% of both groups, with only a few high-grade cases in each, and no significant difference. PSA rose slightly more on testosterone, and urinary symptom scores changed about the same in both groups.
Men with a PSA above 3.0 or severe urinary symptoms were not enrolled, so the result does not cover them. Prostate cancer risk assessment and monitoring remain part of treatment.
In June 2026, HHS asked the FDA to narrow the prostate cancer warning on testosterone labels to metastatic disease and to revise the urinary-symptom warning for mild to moderate enlarged prostate. These are requested changes, not final labels.
Everyday side effects
For testosterone injections, including cypionate, MedlinePlus lists side effects to tell your doctor about if they are severe or do not go away: acne, pain or bruising where you inject, tiredness, trouble sleeping, mood swings, weight gain, headache, and joint or back pain.
Call a doctor right away for leg swelling, trouble breathing during sleep, an erection that will not go away, problems urinating, yellowing skin or eyes, mood changes, or enlarged or painful breasts.
Testosterone is a controlled substance. Higher doses, or use not directed by a doctor, can cause serious side effects.
Who should not start
The Endocrine Society recommends against starting testosterone in men with any of these, without further evaluation:
- Plans to have children in the near term
- An elevated hematocrit
- Untreated severe obstructive sleep apnea
- Severe lower urinary tract symptoms
- Uncontrolled heart failure, or a heart attack or stroke in the last 6 months
- A blood-clotting disorder (thrombophilia)
- Breast or prostate cancer, or a raised PSA that has not been evaluated
Lab checklist
Before and during TRT: five checks
Check blood pressure first
The FDA says all testosterone products raise blood pressure. Know your baseline before you start.
Get hematocrit on a schedule
Before starting, then at 3, 6, and 12 months. Above 54%, the US guideline advises stopping.
Settle the fertility question
Testosterone lowers sperm production. Decide about children before the first prescription.
Ask about the form
Injections raise hematocrit more than gels. Ask how the form you are offered affects monitoring.
Know the urgent symptoms
Leg swelling, breathing trouble during sleep, or an erection that will not go away: call a doctor.
Fit check
Who this guide is (and is not) for
Good fit
- Men weighing testosterone therapy who want the side effects laid out with numbers.
- Men already on TRT who want to know what their monitoring labs are for.
- Men who heard testosterone causes heart attacks and want the newer trial evidence.
Poor fit
- Anyone with chest pain, leg swelling, sudden shortness of breath, or an erection that will not go away. Get medical help now.
- Men looking for a dose or a dosing plan. Your prescriber sets that.
- Men planning children soon. Talk with a doctor before any testosterone.
Common questions
The questions worth answering
Does TRT cause heart attacks?
In the TRAVERSE trial of 5,246 men at high heart risk, heart attacks, strokes, and cardiovascular deaths ran 7.0% on testosterone gel and 7.3% on placebo over about 33 months. The trial did see more atrial fibrillation and pulmonary embolism.
What are the most common side effects of testosterone injections?
MedlinePlus lists acne, injection-site pain or bruising, tiredness, trouble sleeping, mood swings, weight gain, headache, and joint or back pain. Injections also raise red blood cell counts more than gels.
Does TRT make you infertile?
Testosterone therapy lowers sperm production, especially at high doses. The Endocrine Society advises against starting it if you plan to have children soon. Talk with a doctor before you start.
Does TRT raise blood pressure?
Yes. The FDA says blood pressure studies confirmed an increase across all testosterone products, and in 2025 it added a blood pressure warning to labels that lacked one.
What hematocrit is too high on TRT?
The US guideline advises stopping testosterone above a hematocrit of 54%, per a 2017 CMAJ review. A single number is not a diagnosis, so your prescriber reads it with your symptoms and dose.
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