How each works
Intramuscular (IM) deposits testosterone in muscle tissue (typically glute, vastus lateralis, or deltoid). Absorbed via muscle vasculature.
Subcutaneous (SubQ) deposits in the fatty tissue layer just under the skin (typically abdomen or upper thigh). Absorbed via the slower vasculature of fat.
Both work because testosterone esters (cypionate, enanthate) are dissolved in oil, the oil acts as a depot, releasing testosterone over days regardless of which tissue holds it.
Absorption equivalence
Multiple studies confirm SubQ produces equivalent or even slightly better serum testosterone than IM at the same dose. Spratt et al. compared 50 mg weekly SubQ vs IM and found equivalent total T levels. Kaminetsky et al. showed steady-state testosterone is reached at the same timeframe.
Peak-trough smoothness
SubQ produces flatter pharmacokinetics, smaller initial peak, more sustained release. Translation: less peak-related water retention, less estradiol spike, less mood lability between injections. For most men, this means a smoother experience.
Comfort and adherence
| Factor | Subcutaneous | Intramuscular |
|---|---|---|
| Needle size | 27-30 gauge, 1/2-5/8 inch | 22-25 gauge, 1-1.5 inch |
| Injection pain | Minimal | Moderate; can be sore |
| Self-administration | Easy | Doable but harder for glute |
| Bleeding/bruising | Rare | More common |
| Site rotation | Abdomen, thigh | Glute, thigh, deltoid |
SubQ technique
- Needle: 27-29 gauge, 1/2-5/8 inch insulin-style syringe
- Site: abdomen (1-2 inches lateral to navel) or upper thigh
- Pinch fat: grasp a fold of fatty tissue between thumb and finger
- Insert at 45° or 90° depending on body fat amount
- Aspirate (briefly pull back) to confirm not in vessel
- Inject slowly, 30-60 seconds for the volume
- Apply mild pressure after withdrawal
- Rotate sites each injection
When IM is still right
- High-volume doses (more than ~0.5 mL); IM tolerates larger volumes
- Patients with very low body fat (less SubQ tissue available)
- Allergic skin reactions at SubQ sites
- Patient preference for less frequent injections
- Specific clinical situations directed by physician
The clinical pearl: SubQ has become the default modern TRT route for good reasons, equivalent absorption, smoother kinetics, smaller needles, easier self-administration. Most patients should start there unless there's a reason not to.
Bottom line
Subcutaneous testosterone is the modern standard for most patients. It produces equivalent serum testosterone with smoother kinetics, less discomfort, and easier self-administration. IM remains useful in specific situations but is no longer the default. The practical experience for most patients on SubQ TRT is meaningfully better than the older IM-every-two-weeks protocols.