s.lundgren, post #12: Medullary thyroid carcinoma is the reason these compounds are contraindicated in people with personal or family history of MTC or multiple endocrine neoplasia type 2. Rodent toxicology showed a signal; human evidence of causation is absent but caution is appropriate.
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Coming back to post #32, because the follow-up matters more than the original answer.
Allergic reactions: true allergy (anaphylaxis, severe rash) is rare with these peptide compounds but possible. Distinguishing true allergy from GI intolerance matters.