Retatrutide side effects (UK research summary): GI, HR, and titration data

The most-searched UK question about retatrutide is simple: what are the side effects? In Jastreboff et al. (NEJM, 2023), the Phase 2 obesity study reported gastrointestinal events as the dominant tolerability signal — nausea (up to 45% in the 12 mg arm), diarrhoea (~27%), vomiting (~20%) and constipation (~15%). Rates rose with dose and clustered around each titration step.

A smaller but consistent signal was a modest increase in resting heart rate of roughly 3–7 bpm on average, attributed to the glucagon-receptor component. Injection-site reactions were mild and uncommon. No unexpected serious adverse events emerged at 48 weeks.

For UK researchers the practical implication is to stay as low as possible for as long as possible. The schedule used across this library is 2 mg weekly for weeks 1–4, 3 mg for weeks 5–6 and 4 mg for weeks 7–8, stepping up by a single milligram only when the current dose has produced no side effects and appetite suppression has faded — set out in the retatrutide dosage UK guide, with a U-100 syringe unit matrix for 10 mg vials. Reconstitution guidance sits in the reconstitution guide.

Retatrutide has no MHRA marketing authorisation in the UK. It is supplied for laboratory research only. See the retatrutide UK guide for legal status and the full mechanism review.