What size needle is recommended for subcutaneous injection of 2mg Retatrutide in an adult?

Needle Size for Reconstituting Retatrutide 2mg

For reconstituting subcutaneous medications like retatrutide, use a standard reconstitution needle (typically 18-21 gauge) to draw up the diluent and reconstitute the medication, then switch to a 4-mm pen needle or short subcutaneous needle for the actual injection.

Reconstitution vs. Administration: Two Different Needles

The question asks about reconstitution, which is distinct from administration and requires clarification:

For Reconstitution (Drawing Up/Mixing)

• Use a larger gauge needle (18-21 gauge) for withdrawing diluent and reconstituting lyophilized medications, as this allows efficient fluid transfer without excessive pressure 1
• This needle is discarded after reconstitution and never used for injection

For Subcutaneous Administration After Reconstitution

• Use 4-mm pen needles inserted at 90 degrees for all adults regardless of age, sex, ethnicity, or BMI 2
• This recommendation applies to all subcutaneous medications including GLP-1 receptor agonists and similar peptide therapies 2
• The 4-mm needle length is effective and well-tolerated even in adults with obesity 2

Clinical Context for Retatrutide

Retatrutide is administered subcutaneously as a once-weekly injection for obesity and type 2 diabetes 3, 4:

• Doses studied range from 0.5 mg to 12 mg weekly 3, 4
• The medication acts as a triple receptor agonist (GIP, GLP-1, and glucagon receptors) 3, 4
• Administration follows the same principles as other subcutaneous peptide therapies

Injection Site Selection

Recommended subcutaneous injection sites 2:

• Abdomen (2 adult fingerbreadths away from umbilicus)
• Upper third anterior lateral aspect of both thighs
• Posterior lateral aspect of both upper buttocks and flanks
• Middle third posterior aspect of the upper arm

Truncal sites (abdomen and buttocks) may be preferable to limb sites as they have thicker subcutaneous tissue providing a larger margin of safety 5

Critical Safety Considerations

Avoid intramuscular injection 2:

• Inadvertent IM injection leads to unpredictable absorption and variable glucose effects 2
• Risk of IM injection increases with longer needles and injection into limbs rather than truncal sites 2
• The 4-mm needle minimizes this risk across all body types 2

Avoid areas with lipohypertrophy or damaged tissue 5:

• Injection into these areas results in unpredictable absorption
• Rotate injection sites at all times to prevent lipohypertrophy 2

If using needles longer than 4 mm, a correctly lifted skinfold must be used to avoid IM injection 2