Testosterone injections have traditionally been given intramuscularly, and this remains the most common route in the United States. The standard formulations, testosterone cypionate and testosterone enanthate, are dissolved in oil and injected deep into muscle tissue, where they form a slow-release depot. However, subcutaneous injection (into the fat layer just beneath the skin) has become an increasingly popular alternative that uses the same medication.
How Intramuscular Injection Works
When testosterone dissolved in oil is injected into a muscle, it creates a small reservoir at the injection site. The oil-based solution absorbs slowly from the muscle into your bloodstream over days to weeks. Testosterone cypionate, for example, has a half-life of about eight days when given intramuscularly, which is why injections are typically scheduled every one to two weeks.
The most common injection sites are the gluteal muscles (upper outer buttock) and the vastus lateralis (outer thigh). Clinicians typically use a 1 to 1.5 inch needle to reach deep enough into the muscle. For the long-acting formulation testosterone undecanoate, which is given every 10 to 14 weeks, nurses use a larger gauge needle to deliver a full 4 mL of fluid in a single shot.
Why Levels Fluctuate With IM Injections
One well-known drawback of intramuscular testosterone is the roller-coaster pattern in blood levels. After a single injection, testosterone rises into a supraphysiological range (above normal) within the first 48 hours, then gradually drops back down. If the dosing interval is longer than one week, levels often fall below the normal range in the days before the next injection. This peak-and-valley pattern can cause noticeable fluctuations in mood, energy, and libido, which the Endocrine Society lists as a formulation-specific side effect.
Smaller, more frequent doses help smooth out these swings. Splitting a biweekly dose of 150 to 200 mg into weekly injections of 75 to 100 mg keeps levels more stable, though it means twice as many injections.
Subcutaneous Injection as an Alternative
Subcutaneous injection delivers the same testosterone cypionate or enanthate into the fat layer just below the skin, usually in the abdomen or thigh. This route uses shorter, thinner needles and is significantly easier to self-administer. Research published in the Journal of the Endocrine Society found that when testosterone cypionate is injected subcutaneously once a week, mean testosterone levels stay stable and within the normal range between doses, with no local reactions at the injection site.
Patient preference data consistently favors the subcutaneous route. Studies of transgender men who switched from intramuscular to subcutaneous injections found that none wanted to switch back. The main advantages are less pain, no risk of accidentally hitting the sciatic nerve, and the ability to self-inject without help. Intramuscular injections are difficult enough to self-administer that many patients rely on a family member or clinic visit, which creates a real barrier to staying on treatment. Roughly 69% of men prescribed long-acting intramuscular testosterone esters stop treatment within three months, and 95% discontinue within a year.
Subcutaneous injection is not yet FDA-approved specifically for testosterone cypionate or enanthate (these products are labeled for intramuscular use), but clinicians increasingly prescribe it off-label based on the accumulating evidence. The Endocrine Society’s clinical practice guideline encourages clinicians to consider patient preference and treatment burden when choosing a formulation.
Different Formulations, Different Schedules
Not all injectable testosterone follows the same timeline. The schedule depends on the specific ester (the chemical modification that controls how fast the drug releases):
- Testosterone propionate: Short-acting, requiring one or two injections per week.
- Testosterone cypionate or enanthate: Medium-acting, typically injected every one to two weeks.
- Testosterone undecanoate: Long-acting, given every 10 to 14 weeks after an initial loading period. The first dose is followed by a second at six weeks, then regular injections settle into a roughly 12-week cycle. About 70% of patients land on an interval between 10 and 14 weeks.
Testosterone undecanoate is always given intramuscularly by a healthcare provider, in part because the large injection volume isn’t practical for subcutaneous delivery, and in part because of a small risk of a serious coughing reaction (pulmonary oil microembolism) that requires post-injection monitoring.
Risks Specific to Intramuscular Testosterone
Beyond the peaks and valleys in hormone levels, intramuscular injections carry a few specific risks. Pain at the injection site is common. Some people experience a sudden coughing episode immediately after injection, caused by tiny amounts of oil entering the bloodstream and reaching the lungs. This is alarming but typically resolves within minutes.
A broader risk of testosterone therapy regardless of route is erythrocytosis, an increase in red blood cell production that thickens the blood. Symptoms include headache, fatigue, blurred vision, and tingling sensations. If hematocrit (the percentage of blood volume occupied by red blood cells) rises above 54%, testosterone is generally stopped and blood removal may be needed to reduce viscosity. The FDA has issued warnings about the associated risk of blood clots, heart attack, and stroke. Intramuscular injections tend to produce higher hematocrit levels than topical formulations like gels or patches, likely because of the sharper spikes in testosterone after each injection.
Standard monitoring includes a hematocrit check before starting therapy, then again at 3, 6, and 12 months.
Choosing Between IM and Subcutaneous
If your provider prescribes injectable testosterone, the choice between intramuscular and subcutaneous delivery comes down to comfort, convenience, and how well you tolerate each method. Intramuscular injection is the traditional, well-studied route with decades of data behind it. Subcutaneous injection offers easier self-administration, less pain, and potentially smoother testosterone levels with weekly dosing. Both deliver the same medication and achieve comparable blood levels when dosed appropriately.
For people who dislike needles or struggle with self-injection, the subcutaneous route with its smaller needle may be the difference between sticking with treatment and abandoning it. Given that nearly all men on intramuscular injections quit within a year, the practical advantages of a less burdensome method matter as much as the pharmacology.