I've done some preliminary searching and reading about the causes of Post Injection-site Pain (PIP).
Causes of PIP vary by medication type ie steroids vs peptides vs others.
With peptides the a likely cause appears to be a small variation of the med's ph from our body's ph.
The medication ph is usually chemically adjusted during the manufacturing process to normalize it - "buffering" so called.
Not buffering the meds, or not buffering accurately, yields a "burning" sensation at the time of injection.
This should diminish rapidly as the injected solution spreads and is diluted in our bodies.
While not desireable, nothing I have yet read suggests that PIP causes serious or lasting harm.
NOTE: this subject deserves more time than I can give it right now.
I'd enjoy hearing from those of you who have more insight, or more time to research it.
Dennis
EDITED: changed "the likely cause" to " a likely cause".
Cause I do not have the chops to make a definite statement on this subject and everyone has to be informed of that. Sorry!