Why Does Wegovy Cause Injection Site Reactions? The Biology Behind Semaglutide Skin Responses

How often does Wegovy cause injection site reactions?

The figure comes from the label's pooled safety data: three randomized, double-blind, placebo-controlled trials in adults with obesity or overweight, with 2,116 people on semaglutide 2.4 mg weekly for up to 68 weeks and 1,261 on placebo. Injection site reactions were reported by 1.4% of the semaglutide group and 1% of the placebo group [1]. The gap is small: most people injecting Wegovy weekly did not report a local reaction, and the rate sat only slightly above placebo.

The label also cautions that adverse reaction rates from trials run under different conditions cannot be compared directly with each other or with what happens in practice [1].

What is known about the cause

Wegovy is given subcutaneously, into the abdomen, thigh or upper arm, and the label says the injection site can be changed without a dose adjustment [1]. We found no study that explains why injection site reactions occur with Wegovy. A review for emergency clinicians groups injection site reactions with hypersensitivity among the adverse events of GLP-1 agonists, alongside reports of anaphylaxis and other rashes [2]. In a 12-week phase 2 dose-finding trial of weekly semaglutide in 415 adults with type 2 diabetes, injection site reactions were few [3]. Neither source says what produces them.

How semaglutide is absorbed after injection

Semaglutide is 89% bioavailable after subcutaneous injection, more than 99% bound to albumin, and its half-life is about a week [1]. The label reports similar exposure whether the dose goes into the abdomen, thigh or upper arm [1]. Semaglutide reaches its peak blood level 1 to 3 days after an injection [1]. Those measurements describe blood levels; they do not say whether the drug causes local irritation.

Dysesthesia: the skin effect the label describes in detail

Separate from injection site reactions, the label lists dysesthesia at 2% on Wegovy 2.4 mg versus 1% on placebo, a reporting category that includes burning sensation, skin burning sensation, pain of skin, allodynia and sensitive skin [1]. In the two 72-week trials of the higher dose, dysesthesia was reported by 22% on 7.2 mg, 6% on 2.4 mg and 0% on placebo; the label says the risk rises with dose and that 18% of affected people on 7.2 mg had not recovered by the end of the trial [1]. The label describes dysesthesia as altered skin sensation such as burning or pain from touch, not as an injection site reaction.

What the label says to watch for

Serious hypersensitivity is the reaction the label highlights. Anaphylaxis and angioedema have been reported after marketing, and the label says to discontinue Wegovy if a serious hypersensitivity reaction is suspected [1]. Angioedema means swelling of deeper tissue such as the lips, face or throat. A prior serious hypersensitivity reaction to semaglutide or to any excipient in Wegovy is a contraindication [1]. The label also states that the frequency of postmarketing events cannot be estimated from voluntary reports [1].