Why Mounjaro Causes Injection Site Reactions: The Biology Behind Tirzepatide Local Skin Response

How common are injection site reactions with Mounjaro?

Mounjaro is a once-weekly subcutaneous injection of tirzepatide, a dual GIP and GLP-1 receptor agonist [2]. The label reports injection site reactions in 3.2% of Mounjaro-treated adults versus 0.4% of placebo-treated adults in its pool of placebo-controlled trials [2]. The Zepbound label, a different tirzepatide product studied in people with obesity or overweight, reports 2% on placebo versus 6%, 8% and 8% on 5 mg, 10 mg and 15 mg.

Is the reaction an antibody response?

An analysis of seven phase 3 trials in type 2 diabetes found treatment-emergent anti-tirzepatide antibodies in 51.1% of 5,025 tirzepatide-treated patients. Neutralizing antibodies against tirzepatide activity at the GIP and GLP-1 receptors were uncommon: 1.9% and 2.1%, respectively [1].

More antibody-positive patients reported hypersensitivity reactions or injection site reactions than antibody-negative patients [1]. The Mounjaro label gives the numbers for injection site reactions: 119 of 2,570 antibody-positive adults (4.6%) versus 18 of 2,455 antibody-negative adults (0.7%) [2]. That is an association, not proof that antibodies cause the local reaction. The analysis also found that most hypersensitivity and injection site reactions were nonserious and nonsevere, and most events occurred or resolved regardless of antibody status or antibody titer [1]. Antibody status, titer and neutralizing status had no effect on tirzepatide pharmacokinetics or efficacy [1].

How does tirzepatide compare with other GLP-1 drugs?

A 2026 network meta-analysis of phase 3 weight-management trials (adults with a BMI of 25 or higher) included 24 trials, 23 of which were eligible for the analysis. It associated tirzepatide with a higher risk of injection site reactions than the comparator GLP-1 drugs, and found no significant between-drug difference in serious adverse events [3]. The analysis does not identify a biological reason for the difference.

What about serious reactions?

The label lists injection site reactions and hypersensitivity reactions as separate adverse reactions; hypersensitivity reactions were reported in 3.2% of Mounjaro-treated adults versus 1.7% on placebo [2]. Its warning covers serious hypersensitivity reactions: anaphylaxis and angioedema have been reported with Mounjaro, known serious hypersensitivity to tirzepatide or any excipient is a contraindication, and the label says to discontinue Mounjaro if a serious hypersensitivity reaction is suspected and promptly seek medical advice [2].