Should I inject peptides subcutaneously or intramuscularly?

Subcutaneous is the default for most peptides including BPC-157, TB-500, Ipamorelin, CJC-1295, semaglutide, and tirzepatide. IM is primarily for large-volume preparations like Cerebrolysin. Unless a protocol specifically calls for IM, SubQ with an insulin syringe is standard.

What is the difference in absorption between SubQ and IM?

SubQ has slower absorption (peak 1-4 hours, 70-90% bioavailability). IM is faster (peak 30-60 min, 90-100% bioavailability). Slower SubQ absorption is actually desirable for many peptides as it provides sustained release.

What needle size for peptide injections?

SubQ: 29-31 gauge, 1/2 inch (insulin syringes). IM: 23-25 gauge, 1-1.5 inches. Higher gauge = thinner needle = less pain.

Where to inject peptides subcutaneously?

Abdomen (2+ inches from navel) is most common. Outer thigh and back of upper arm are alternatives. For BPC-157, inject near the injury site. Rotate between sites.

Is IM more effective than SubQ for peptides?

Not necessarily. IM has higher bioavailability and faster absorption, but many peptides benefit from SubQ's slower sustained release. GLP-1 agonists are specifically designed for SubQ. IM is better only when rapid absorption or large volumes are needed.

Subcutaneous vs Intramuscular Injection: When to Use Each for Peptides

Why Injection Route Matters for Peptides

Subcutaneous (SubQ) and intramuscular (IM) injection deliver peptides differently — affecting absorption rate, bioavailability, and peak concentration. This guide covers when to use each. For complete preparation, see our Reconstitution and Injection Guide.

Subcutaneous: The Default for Most Peptides

SubQ injection delivers peptide into the fat layer between skin and muscle. The peptide forms a depot that absorbs gradually into capillaries.

Absorption: Moderate — peak in 1-4 hours
Bioavailability: 70-90%
Volume: Best for 0.1-1.0 mL
Needle: 29-31 gauge, 1/2 inch (insulin syringe)
Pain: Minimal
Sites: Abdomen (2 inches from navel), outer thigh, upper arm

SubQ Technique

Clean site with alcohol swab
Pinch a fold of skin
Insert needle at 45-90 degrees
Inject slowly, release pinch, withdraw, apply pressure

Intramuscular: For Specific Compounds

IM injection delivers peptide into muscle tissue with its rich blood supply, producing faster absorption.

Absorption: Fast — peak in 30-60 minutes
Bioavailability: 90-100%
Volume: 1-5 mL depending on muscle
Needle: 23-25 gauge, 1-1.5 inches
Pain: More than SubQ
Sites: Deltoid, vastus lateralis (outer thigh), ventrogluteal (hip)

Head-to-Head Comparison

Factor	Subcutaneous	Intramuscular
Peak time	1-4 hours	30-60 minutes
Bioavailability	70-90%	90-100%
Needle gauge	29-31G	23-25G
Pain	Minimal	Moderate
Max volume	~1 mL	2-5 mL
Self-injection	Easy	Harder for some sites
Absorption pattern	Gradual, sustained	Rapid peak

Which Peptides Use Which Route?

Peptide	Route	Rationale
BPC-157	SubQ (near injury)	Local concentration advantages
TB-500	SubQ	Systemic distribution sufficient
Semaglutide	SubQ	FDA protocol; slow absorption desired
Tirzepatide	SubQ	FDA protocol
Ipamorelin	SubQ	Standard secretagogue route
CJC-1295	SubQ	Depot effect desired for DAC version
Cerebrolysin	IM or IV	Large volumes (5-30 mL)
Tesamorelin	SubQ (abdomen)	FDA-approved abdominal SubQ
Sermorelin	SubQ	Standard protocol

General rule: If dosed in micrograms with small vials, SubQ with an insulin syringe is correct. IM is for large-volume preparations or when rapid absorption is specifically needed.

Special Cases

Local Injection (BPC-157)

BPC-157 benefits from injection near the injury — still subcutaneous, not into the tendon/joint. See the BPC-157 Dosage Guide.

GH Secretagogues

Ipamorelin and CJC-1295 are injected SubQ before bed. The slower absorption provides sustained stimulus aligned with the nocturnal GH pulse.

GLP-1 Agonists

Semaglutide and tirzepatide are designed for SubQ. The slow absorption contributes to weekly duration. IM would increase peak side effects while reducing duration.

Safety Tips

Rotate injection sites to prevent lipodystrophy (SubQ) or fibrosis (IM)
Never reuse needles
Aspirate before IM injection (check for blood)
Use bacteriostatic water for multi-dose vials
Use puncture-resistant sharps containers for disposal

For complete preparation, see How to Reconstitute Peptides. For dose calculations, use the Reconstitution Calculator.