Testosterone Replacement Therapy (TRT) restores testosterone levels in individuals diagnosed with hypogonadism. Injectable testosterone, commonly called “shots,” is a primary method of administration. The duration a shot remains effective is not fixed; it depends entirely on the specific chemical formulation administered. This formulation controls the absorption rate and determines the prescribed injection frequency, which can range from a few days to several weeks.
Understanding Testosterone Esters
The core factor determining how long a testosterone injection lasts is the ester attached to the testosterone molecule. Testosterone itself is rapidly metabolized, so it cannot be given in its pure form as a long-acting injection. The ester is a fatty acid chain bonded to the testosterone, making the compound lipid-soluble and allowing it to be suspended in an oil base. Once injected into the muscle, this oil depot slowly releases the testosterone-ester compound into the bloodstream.
The length of the ester chain directly influences the release rate. Shorter ester chains, such as testosterone enanthate, are broken down more quickly by enzymes, resulting in a faster release of the active hormone. Longer ester chains, like testosterone undecanoate, dissolve slower from the injection site, leading to a more gradual and sustained release. The body must break this ester bond to free the biologically active testosterone hormone.
Typical Injection Schedules and Duration of Effect
Testosterone cypionate and testosterone enanthate are the most commonly prescribed medium-acting esters. They have similar release profiles and are typically administered every seven to fourteen days. To prevent undesirable peaks and subsequent drops, many clinicians prefer a more frequent schedule, such as every five to seven days.
These protocols create a peak concentration within 48 to 72 hours following the injection. The therapeutic duration lasts until the concentration falls to the “trough,” the lowest hormone level just before the next dose. Intervals longer than two weeks can cause hormone concentrations to dip below the necessary therapeutic range.
Ultra-Long Acting Esters
Testosterone undecanoate is an ultra-long-acting ester designed for extended intervals. It is often administered following an initial loading dose to quickly establish therapeutic levels. After this phase, a single injection can maintain hormone levels for ten to twelve weeks. This extended duration is ideal for those who prefer less frequent injections.
Recognizing the End of the Therapeutic Window
The therapeutic window ends when the testosterone concentration falls to the trough level just before the next scheduled injection. As the ester is metabolized and active testosterone is cleared, the patient may experience a recurrence of the original symptoms of low testosterone.
Physical Symptoms
Physiological signs often include increased fatigue and a reduction in muscle mass or strength. Patients may also report a decline in sexual desire and function, including issues with maintaining an erection.
Psychological Symptoms
Psychological symptoms can be prominent, manifesting as changes in mood and cognitive function. Irritability, increased emotional sensitivity, and a depressed mood are commonly reported as the trough approaches. Individuals may also note difficulty with concentration and memory, often described as a lack of mental clarity. These symptoms, sometimes called “the crash,” signal the necessity of the next dose.