GLP-1 Injection Site Reactions: Complete Guide to Side Effects, Nausea, Constipation, and Sulfur Burps

What an injection site reaction looks like

A 2019 review of injection site reactions to self-injected biologic medicines, written mainly around products for skin conditions, defines them as swelling, redness (erythema), itching and pain around the injection point [3]. That review is not specific to GLP-1 drugs.

For GLP-1 drugs, a 2014 review of the class lists injection site reactions alongside headache and nasopharyngitis as common adverse effects that do not usually result in discontinuation [1]. A 2024 review for emergency clinicians notes that hypersensitivity, including injection site reactions, has been associated with GLP-1 agonist use, with reports of anaphylaxis and other rashes [5].

The product labels are quieter on this point than the reviews. In the Wegovy (semaglutide) label, the table of adverse reactions reported in at least 2% of treated patients and more often than placebo is dominated by gastrointestinal effects, and injection site reactions are not in it [2].

How the labels describe injecting

Wegovy is injected subcutaneously once weekly, on the same day each week, at any time of day, with or without meals, into the abdomen, thigh, or upper arm, and the label says to rotate injection sites with each dose [2]. The label says to inspect the injection before each administration and use it only if the solution is clear, colorless and free of particles [2]. It also says patients should receive training appropriate to the specific Wegovy presentation they will use, and that the FlexTouch pen must never be shared between patients, even if the needle is changed [2].

When a site reaction is more than a site reaction

The Wegovy label lists serious hypersensitivity reactions, including anaphylaxis and angioedema, as reported with the drug, and a prior serious hypersensitivity reaction to semaglutide or to any excipient is a contraindication to using it [2]. The label says to discontinue Wegovy and promptly seek medical advice if a hypersensitivity reaction is suspected [2].

Nausea: the reaction the trials record most

Wegovy 2.4 mg weekly, over 68 weeks: nausea 44% versus 16% on placebo, diarrhea 30% versus 16%, vomiting 24% versus 6%, constipation 24% versus 11% [2]. Permanent discontinuation because of adverse reactions was 6.8% versus 3.2%, and the most common reasons were nausea (1.8% versus 0.2%), vomiting (1.2% versus 0%) and diarrhea (0.7% versus 0.1%) [2].

Tirzepatide (Zepbound) 15 mg, pooled across trials of up to 72 weeks: nausea 28%, vomiting 13%, diarrhea 23% and constipation 11%, against 8%, 2%, 8% and 5% on placebo [4].

Orforglipron (Foundayo) 17.2 mg, a once-daily tablet rather than an injection, in a 72-week trial pool: nausea 35%, vomiting 24%, diarrhea 25% and constipation 24%, against 10%, 4%, 11% and 9% on placebo [6]. Gastrointestinal adverse reactions of any kind occurred in 69% at 17.2 mg versus 37% on placebo, and 6% versus 0.7% stopped treatment because of them [6]. That label also states that the incidence of nausea, vomiting and diarrhea was higher during the dose-escalation period and decreased over time [6].

Those figures should not be ranked against each other: the labels state that adverse reaction rates from clinical trials conducted under widely varying conditions cannot be directly compared [2][6].

Constipation and dehydration

Constipation is among the most frequent reactions: 24% versus 11% with Wegovy 2.4 mg [2], 11% versus 5% with Zepbound 15 mg [4], and 24% versus 9% with Foundayo 17.2 mg [6]. It appears in the list of most common adverse reactions (5% or more) for both Wegovy and Foundayo [2][6].

The labels also address dehydration. Wegovy warns about acute kidney injury due to volume depletion and says to monitor renal function in patients reporting reactions that could lead to volume depletion [2]. The Foundayo label says most reported kidney injury events occurred in patients with gastrointestinal reactions that led to dehydration, such as nausea, vomiting or diarrhea [6].

Belching and sulfur burps

Burping is a recorded trial outcome, under the label term eructation. Wegovy: 7% versus less than 1% on placebo [2]. Foundayo: 8% at 17.2 mg versus 1% [6]. Abdominal distension and flatulence are listed too: 7% versus 5% and 6% versus 4% with Wegovy [2].

The adverse reaction tables use the term eructation and do not describe an odor [2][6]. Both labels state that these drugs delay gastric emptying [2][6]. A 2021 review of the class notes that the effects of GLP-1 receptor agonists on gastric emptying decrease over time, a tachyphylaxis effect [7].

What the labels flag as serious

Pancreatitis. The Wegovy label says to observe patients for persistent or severe abdominal pain, sometimes radiating to the back, which may or may not come with nausea or vomiting, and to discontinue the drug if pancreatitis is suspected [2]. The Foundayo label carries the same instruction [6].

Severe gastrointestinal reactions. In the orforglipron trials, severe gastrointestinal reactions were reported by about 3% of treated patients versus 1% on placebo [6]. Neither label recommends these drugs in patients with severe gastroparesis [2][6], and the labels do not say that GLP-1 drugs cause gastroparesis [2][6].

Gallbladder disease. In Wegovy's adult weight-management trials, cholelithiasis was reported by 1.6% versus 0.7% on placebo and cholecystitis by 0.6% versus 0.2% [2].

Hypoglycemia. The labels say the risk rises when these drugs are combined with insulin or a sulfonylurea, and suggest considering a lower dose of the insulin or sulfonylurea when starting [2][6].

Heart rate. The Wegovy label reports mean resting heart rate increases of 1 to 4 beats per minute [2].

Pulmonary aspiration. The Wegovy label reports pulmonary aspiration in patients receiving GLP-1 receptor agonists who underwent elective surgeries or procedures under general anesthesia or deep sedation, and says to instruct patients to inform healthcare providers of any planned surgeries or procedures [2].

Thyroid C-cell tumors. Wegovy carries a boxed warning based on rodent thyroid C-cell tumors, with human relevance unknown, and is contraindicated in people with a personal or family history of medullary thyroid carcinoma or MEN 2 [2].

Do the tablets avoid this problem

Four FDA-approved GLP-1 products are tablets rather than injections: Wegovy tablets (semaglutide 25 mg once daily, taken on an empty stomach with up to 4 ounces of water, 30 minutes before food or other oral medicines) [2], Foundayo (orforglipron 0.8 mg once daily, stepped up to a maximum of 17.2 mg, with or without food) [6], and Rybelsus and Ozempic tablets, both oral semaglutide for type 2 diabetes [8]. None of the four is injected, so injection site reactions do not apply to them. Ozempic is also sold as a once-weekly injection [9].