Route selection is pharmacokinetic, not personal preference. Oil-based AAS require IM administration because the depot behaviour depends on intramuscular vascularity and esterase activity profiles at the injection site. Aqueous peptides and low-concentration lipophilic compounds are better suited to subcutaneous administration because the slower absorption from adipose produces flatter serum curves with less injection trauma per dose.
Intramuscular (IM)
Oil deposited between muscle fibres creates a depot. Local esterase activity hydrolyses the ester bond progressively, releasing free hormone into venous return. Bioavailability at properly-placed IM injection: 92–98%.
Indicated for:
- All oil-suspended AAS: testosterone esters (E/C/P/U), nandrolone decanoate and phenylpropionate, boldenone undecylenate, trenbolone esters, drostanolone, methenolone enanthate.
- Aqueous testosterone suspension. Crystalline suspension requires muscle vascularity; SubQ fails.
- hCG at doses >500 IU per injection (volume consideration).
Needle specifications: 23G × 1″ to 25G × 1.5″ depending on site and subcutaneous adipose depth. Glutes and quads tolerate 1.5″; deltoid 1″ is standard.
Sites, ranked by suitability: ventrogluteal (upper-outer gluteal quadrant) is the first-line site — large muscle mass, minimal neurovascular hazard, reliable depot formation. Vastus lateralis (outer-middle quad) is the second choice, good for volumes up to 2 mL. Deltoid (lateral head only — never anterior or posterior, sciatic and axillary nerve proximity) accepts volumes up to 1 mL.
Pectoral injection is documented in some training cultures but carries elevated risk of pneumothorax in lean subjects (thin chest wall) and proximity to internal thoracic artery. Not recommended outside supervised settings.
Subcutaneous (SubQ)
Deposit into subdermal adipose. Absorption is slower and steadier than IM, mediated by capillary diffusion rather than muscle perfusion. Bioavailability: 85–95%, marginally below IM, with slower absorption producing flatter peak-to-trough profile.
Indicated for:
- All lyophilised peptides post-reconstitution: BPC-157, TB-500, Ipamorelin, GHRP-2/6, CJC-1295, Sermorelin, Tesamorelin.
- Somatropin (HGH) — the route for which SubQ was pharmacokinetically optimised.
- hCG at doses ≤500 IU.
- Insulin (diabetic-class indications are universally SubQ).
- Water-based AAS (Winstrol suspension) — technically viable, but PIP from the crystalline suspension in SubQ tissue is severe. IM is preferred despite the same active ingredient.
Needle specifications: 29G or 31G insulin syringe, 0.5″ (12.7 mm) or 0.625″ (15.9 mm). Thin needle, minimal trauma.
Sites: abdominal subcutaneous tissue (lateral to the umbilicus, avoiding a 2-inch perimeter around the navel where vascular and fascial structure differs), lateral thigh, and the “love handle” flank region. Pinch the tissue to lift the adipose away from underlying muscle; inject at 45°.
Subcutaneous testosterone — the evidence
Kovac et al. 2014 and Spratt et al. 2017 established SubQ testosterone enanthate/cypionate as pharmacokinetically equivalent to IM for TRT purposes. Serum profile at matched weekly dose is slightly flatter with SubQ (lower peak, marginally higher trough), and patient-reported injection pain is substantially lower.
Constraints that apply:
- Volume ceiling. SubQ practically tops out at 1 mL per injection. Volumes above this produce an uncomfortable bleb and absorption slows disproportionately. Twice-weekly dosing at 0.5 mL each keeps the protocol within tolerance.
- Site nodules. A minority of users develop subcutaneous injection-site nodules with repeated SubQ testosterone. Strict site rotation (7+ day interval between same-site injections) reduces incidence. Nodules that do form typically resolve within 4–6 weeks off the site.
- Oil viscosity at low temperature. Thick carriers (cottonseed) are harder to inject through 29G insulin needles. Grapeseed or MCT carriers are better suited to SubQ testosterone.
Route errors
Oil-based AAS injected SubQ: oil disperses poorly in adipose tissue; lumps persist for weeks; absorption is erratic. Never indicated. Ambient conditions where this happens: accidental needle angle under 45° on a SubQ technique intended for IM.
Aqueous peptides injected deep IM: pointless tissue trauma with no pharmacokinetic benefit. Peptides do not depot in muscle tissue; SubQ produces equivalent or superior serum profile at lower injection cost.
Practical note on syringe acquisition
Pharmacy counters across most EU jurisdictions dispense IM and insulin syringes without prescription. Regulatory treatment varies by country; direct-to-patient sale is the norm in Germany, Poland, Netherlands, UK. Bulk pharmacy-grade syringes are preferable to online suppliers — the quality control on needle sharpness and lubricant coating is measurably better at pharmacy source.