Test Undecanoate: Oral vs. Injectable Testosterone

Testosterone undecanoate is a long-chain ester of testosterone used to treat low testosterone in adult men, available in both injectable and oral forms. It stands apart from other testosterone preparations because of how the body absorbs it: instead of passing through the liver and being rapidly broken down, it travels through the lymphatic system, which dramatically changes its pharmacokinetics and allows for much longer intervals between doses. Injectable testosterone undecanoate is marketed under the brand names Aveed (in the United States) and Nebido (in many other countries), while oral versions include Jatenzo, Tlando, and Kyzatrex.

How Testosterone Undecanoate Gets Into the Body

Most oral testosterone gets destroyed almost immediately. When you swallow plain testosterone, it passes from the gut into the portal vein and straight to the liver, which metabolizes it so thoroughly that almost none reaches the bloodstream in active form. This so-called first-pass effect is why testosterone pills were historically considered impractical.

Testosterone undecanoate sidesteps this problem. The undecanoate ester is a long fatty-acid chain attached to the testosterone molecule, making the compound highly fat-soluble. Because of this fat solubility, a significant portion of TU is absorbed through the lymphatic vessels in the intestinal wall rather than through the portal blood supply. Research in humans confirmed that the main compounds appearing in lymph after oral dosing were TU itself and a reduced metabolite, and that this lymphatic route accounted for meaningful systemic testosterone delivery.¹ Animal studies later demonstrated that this lymphatic transport produces increased systemic exposure to testosterone precisely by avoiding the liver’s first-pass destruction.²

This mechanism has a practical consequence: because TU hitches a ride on the same lymphatic pathways that absorb dietary fat, what you eat alongside the oral capsule matters. Newer self-emulsifying formulations have reduced the meal dependency somewhat, but taking oral TU with food still produces substantially higher testosterone levels than taking it on an empty stomach. In pharmacokinetic testing, fasting resulted in roughly half or less of the cumulative testosterone exposure compared to taking the capsule with a meal containing a moderate amount of fat.³ Even a very-low-fat meal improved absorption over fasting. As a general rule, you get better results taking oral TU with a regular meal rather than on an empty stomach, though the newer formulations are more forgiving about exactly how much fat is on the plate.

Injectable Versus Oral Forms

The injectable form of TU is dissolved in castor oil and given as a deep intramuscular injection, typically into the gluteal muscle. After the initial loading phase (two injections spaced about six weeks apart), it only needs to be given roughly every ten to fourteen weeks, or about four times a year. That makes it the longest-acting injectable testosterone formulation widely available.⁴ By comparison, the more commonly used testosterone cypionate and testosterone enanthate injections are usually given every one to two weeks.

The pharmacokinetic profile of injectable TU is notably smoother than those shorter-acting esters. Testosterone cypionate and enanthate tend to produce a sharp spike shortly after injection followed by a trough as the next dose approaches, which some men notice as mood and energy fluctuations. Injectable TU does not produce the same supraphysiological peaks, and the lowest levels occur later in the dosing interval rather than right before the next shot.⁵ In clinical studies, the average serum testosterone following injection settled around 495 ng/dL over the dosing interval, with about 94% of patients maintaining average levels within the normal range for young healthy men.⁶

The oral form trades that long duration for convenience of a different kind: no needles. Newer oral TU capsules (Jatenzo, Tlando, Kyzatrex) use a self-emulsifying drug-delivery system that improves absorption and avoids the liver toxicity historically associated with older oral androgens like methyltestosterone.⁷ These capsules are taken twice daily with meals, so they require more frequent dosing but avoid injections entirely.

The POME Risk With Injections

The most distinctive safety concern with injectable TU is something called pulmonary oil microembolism, or POME. Because the drug is dissolved in a relatively large volume of castor oil and injected deep into muscle, a small amount of that oil can occasionally enter a blood vessel and travel to the lungs. When this happens, the most common symptom is a sudden coughing episode, sometimes accompanied by chest tightness, dizziness, flushing, or a feeling of shortness of breath.⁸ These reactions are almost always self-limiting, meaning they resolve on their own within minutes, but they can be alarming.

In the United States, the FDA required a Risk Evaluation and Mitigation Strategy (REMS) program for Aveed because of the potential for POME and anaphylaxis after injection.⁹ Under this program, injections must be administered in a healthcare setting, and the patient is monitored for about 30 minutes afterward. This requirement is one of the reasons injectable TU has not replaced shorter-acting esters for every patient: you cannot self-inject it at home the way many men do with testosterone cypionate. In countries where Nebido is used, the monitoring requirements vary, but clinical supervision during injection is generally recommended.

Effects on Body Composition and Bone

Testosterone replacement with TU consistently shifts body composition toward more lean mass and less fat. In men with obesity-related low testosterone, injectable TU therapy led to a reduction in fat mass of about 3.5 kilograms and an increase in lean body mass of roughly 2.9 kilograms.¹⁰ In a larger placebo-controlled trial of oral TU in older men with symptomatic testosterone deficiency, lean body mass increased and body fat decreased in a clear dose-dependent pattern, with effects visible by six months.¹¹

Bone density also responds to TU treatment. In middle-aged men with low testosterone and metabolic syndrome, three years of injectable TU produced a roughly 5% annual increase in bone mineral density at both the spine and hip.¹² Elderly men with osteoporosis and low testosterone also showed significant bone density improvements at the lumbar spine and femoral neck when treated with low-dose oral TU, with gains appearing within six to twelve months.¹³ These bone effects are not unique to TU as a formulation; they reflect the role of testosterone in maintaining bone health. But the sustained, stable levels that TU delivers may contribute to steady improvements over time rather than the fluctuations that come with shorter-acting preparations.

Metabolic and Cardiovascular Outcomes

Some of the most striking data on TU come from long-term registry studies looking at metabolic health. In men with both low testosterone and type 2 diabetes, years of injectable TU therapy led to significant and progressive reductions in fasting glucose, HbA1c, and fasting insulin levels.¹⁴ In one real-world registry following patients for up to twelve years, the treated group saw HbA1c fall progressively while the untreated comparison group’s glucose control worsened, with the gap between the two groups widening over time.¹⁵ The insulin resistance index also dropped substantially in the treated group while climbing in untreated men.

These are observational findings, so they carry the usual caveats about causation versus correlation. Men who stay on testosterone therapy for a decade may differ from those who do not in ways that affect metabolic health. Still, the consistency and magnitude of the metabolic improvements across multiple registries have drawn attention.

On the cardiovascular side, a 24-month randomized trial in men with low testosterone and metabolic syndrome found that TU improved insulin resistance, reduced markers of arterial inflammation, and decreased carotid artery wall thickness compared to placebo. Visceral fat and waist circumference dropped significantly, even though overall body weight did not change much.¹⁶ A separate long-term observational study of men with testosterone deficiency and a history of cardiovascular disease found that sustained TU treatment improved lipid profiles, blood pressure, heart rate, and glycemic control, with no major cardiovascular events during follow-up.¹⁷

Rising Red Blood Cells

Every form of testosterone therapy raises hematocrit, the percentage of your blood volume occupied by red blood cells. TU is no exception, though its effect appears somewhat gentler than shorter-acting injectables. In a systematic review of testosterone formulations used in transgender men, injectable TU raised hematocrit by up to about 5%, compared to up to about 7% with testosterone enanthate.¹⁸ The likely explanation is that TU’s smoother pharmacokinetic profile avoids the supraphysiological testosterone peaks that stimulate red blood cell production most aggressively.

That said, polycythemia (hematocrit rising above the upper limit of normal) is still common with long-term TU therapy. In an observational cohort study tracking men on TU for years, about 40% of patients experienced polycythemia at some point during follow-up, a rate that did not differ between older and younger men.¹⁹ Elevated hematocrit thickens the blood and can increase the risk of clotting events, so regular blood monitoring is standard practice for anyone on testosterone therapy, including TU. If hematocrit climbs too high, the usual response is to reduce the dose, extend the interval between injections, or temporarily hold treatment.

What About the Prostate?

Fear that testosterone therapy drives prostate cancer has lingered for decades, and it remains one of the most common concerns patients raise. The evidence with TU, drawn from multi-year follow-up studies, has been reassuring. In a study spanning more than eight years of continuous TU injections, prostate safety parameters stayed well within reference limits.²⁰ A separate four-year study found that prostate volume increased modestly over the first twelve months (from about 20 mL to 22 mL) and then stabilized, with PSA following the same pattern: a small early rise followed by a plateau.²¹

The initial small increase in prostate size is expected when testosterone levels in a deficient man are restored to normal; it reflects the prostate returning to a physiologically appropriate state rather than overshooting into dangerous territory. The stabilization after the first year, even with ongoing therapy, is the pattern clinicians consider important. Routine prostate screening (PSA tests and digital rectal exams) remains standard for men on any form of testosterone replacement, but the available long-term TU data have not shown the prostate alarm that earlier generations of clinicians feared.

Sexual Function and Quality of Life

For many men, improvements in sexual function are the most noticeable result of testosterone replacement. In a placebo-controlled trial of injectable TU in men with type 2 diabetes and low testosterone, all domains of sexual function improved, including erectile function, desire, orgasm, and satisfaction. Benefits appeared as early as six weeks and continued to build over 18 months. By 30 weeks, nearly half the men on active therapy reported that treatment had improved their health, compared to about one in six on placebo. After everyone switched to open-label TU, the proportion feeling improved reached 70%.²²

Whether testosterone therapy helps with depression is less clear. A review of the evidence found that TU supplementation did not show strong evidence of effectiveness for treating depression specifically, but it did improve sexual function and cognitive function in ways that contributed meaningfully to quality of life in aging men with partial androgen deficiency.²³ The distinction matters: if your low mood is primarily driven by low testosterone, you may feel better when levels normalize, but TU is not an antidepressant and should not be treated as one.

Use in Gender-Affirming Hormone Therapy

Testosterone undecanoate is increasingly used in masculinizing hormone therapy for transgender men and transmasculine people. A randomized controlled trial comparing TU to testosterone enanthate over one year found that both formulations produced the desired masculinizing effects with similar changes in hemoglobin, hematocrit, cholesterol, and hormone levels. The key practical difference was dosing frequency: the TU group received roughly six injections over the year compared to about eighteen for the enanthate group. At the end of the study, participants preferred TU because of the longer intervals between visits.²⁴

Fewer injections mean fewer clinic visits, less disruption to daily life, and potentially better adherence over the long term. For people who experience injection anxiety, cutting the number of shots by two-thirds is meaningful. The tradeoff is that each individual injection of TU involves a larger volume of oil (typically 4 mL versus about 1 mL for enanthate), which can make the injection itself more uncomfortable, and the POME monitoring requirement in the U.S. means each visit takes longer than a quick self-administered shot at home would.

Research Into Male Hormonal Contraception

An area where TU has been explored but has not yet reached clinical use is male hormonal contraception. The idea is straightforward: exogenous testosterone suppresses the brain’s signals to the testes, which in turn suppresses sperm production. Injectable TU’s long action and stable levels made it a candidate for this purpose, and early research suggested it could contribute to a safe, reversible, and effective male contraceptive regimen.²⁵ Most trials have combined TU with a progestin to achieve more reliable suppression of sperm counts. The approach works pharmacologically, but development has been slow due to a mix of side-effect concerns, regulatory caution, and limited commercial interest. No TU-based male contraceptive is approved or close to market as of now.

Cost and Access Considerations

Injectable testosterone in general remains the most cost-effective option for testosterone replacement therapy. Among injectables, TU’s four-times-per-year dosing schedule is appealing on paper, but the medication itself is considerably more expensive per dose than testosterone cypionate or enanthate, and the requirement for in-office administration in the United States adds clinic visit costs. For men who are comfortable with self-injection, a vial of testosterone cypionate administered at home every week or two is often the cheapest route. TU tends to be chosen when the smoother pharmacokinetic profile, the less frequent dosing, or the avoidance of peaks and troughs is clinically important enough to justify the added cost.

The newer oral TU formulations occupy a different niche. They appeal to men who strongly prefer not to inject, but they require twice-daily dosing with meals and carry a higher price tag than generic injectable cypionate. Insurance coverage varies widely. Some plans cover Jatenzo or Tlando as branded medications; others require prior authorization or deny coverage entirely, steering patients toward cheaper injectables or topical gels instead.