# Semaglutide Side Effects: What the Research Literature Reports > Semaglutide side effects from clinical trials: GI events, pancreatitis, thyroid risk, hair loss, and drug interactions — each finding cited from the STEP, SUSTAIN, and SELECT trial programs. ## Adverse Event Profile from the Trial Program Semaglutide's adverse event profile across the STEP, SUSTAIN, and SELECT programs is well-characterized in large randomized populations. Gastrointestinal events are the predominant category; serious adverse events are less common but documented. All adverse event data cited here are from placebo-controlled randomized trials. ## Gastrointestinal Adverse Events Gastrointestinal adverse events — nausea, vomiting, diarrhea, constipation — were reported by up to 74% of semaglutide-treated participants across STEP trials [12]. In STEP 1, approximately 44% reported nausea, 24% diarrhea, 24% vomiting, and 24% constipation in the semaglutide arm versus approximately 16%, 11%, 6%, and 11% respectively with placebo [1]. Severity: most events were mild to moderate. The incidence was highest during dose escalation (weeks 1-20) and declined as participants reached and maintained the 2.4 mg maintenance dose [12]. Discontinuation due to adverse events: 7.0% of semaglutide participants versus 3.1% with placebo in STEP 1 [1]. ## Timing of Semaglutide Side Effects In clinical trials, GI side effects were most frequent during the dose-escalation phase (weeks 1-20 in the STEP titration) [12]. After week 20, event rates in STEP 1 data fell substantially, with most participants no longer reporting active GI symptoms. ## Managing Common Semaglutide Side Effects The STEP titration protocol's slow 16-week escalation schedule represents the primary GI-mitigation strategy validated in large trials [1]. Specific trial-protocol guidance included splitting meals, avoiding high-fat foods at the time of dose escalation, and maintaining adequate fluid intake. ## Serious Adverse Events in Semaglutide Trials **BLACK-BOX WARNING**: Thyroid C-cell tumors were observed in rodent studies at suprapharmacological exposures, generating an FDA black-box warning. No confirmed cases of medullary thyroid carcinoma have been attributed to semaglutide in any human clinical trial population to date [14]. Acute pancreatitis: 0.2% of semaglutide-treated participants versus less than 0.1% with placebo in STEP trial data [12]. Diabetic retinopathy complications were significantly more common with semaglutide than placebo in SUSTAIN-6: 3.0% versus 1.8% (HR 1.76; p=0.02), attributed to the early-worsening phenomenon with rapid HbA1c improvement, not direct drug toxicity [13]. ## What Are the Risks and Downsides of Semaglutide? Most common risks: GI events (nausea, vomiting, diarrhea, constipation), affecting 40-74% of treated participants during dose escalation, mostly mild-to-moderate [12]. 7% treatment discontinuation rate in STEP 1. Serious but less common: pancreatitis (0.2% vs <0.1% with placebo), gallbladder disease, and acute kidney injury (in some participants with severe dehydration from vomiting/diarrhea) [12, 14]. Lean mass loss: approximately 10% absolute lean body mass reduction alongside fat loss in the DXA substudy of STEP 1 [11]. Sarcopenia implications in older adults are an active area of investigation. Long-term safety data exist up to approximately 3.4 years (SELECT trial follow-up) [5]. 5-10 year outcomes data are not yet available. ## Populations Excluded from Semaglutide Research Trials Standard exclusion criteria across STEP and SUSTAIN trials [14]: - Personal or family history of medullary thyroid carcinoma - Multiple endocrine neoplasia type 2 (MEN-2) syndrome - History of acute pancreatitis - Pregnancy Additionally, individuals with end-stage renal disease (eGFR <15 mL/min/1.73 m²) were excluded from most trials. FLOW enrolled patients with CKD (eGFR 50-75 mL/min/1.73 m² at entry) [19]. ## Semaglutide and Hair Loss: Trial Data Alopecia was reported by approximately 3% of semaglutide-treated subjects versus approximately 1% with placebo in STEP program trials [12]. In subjects achieving greater than 20% weight loss, alopecia was reported at 5.3% versus 2.5% [24]. Researchers characterize the pattern as telogen effluvium — temporary, reversible diffuse hair shedding — hypothesized secondary to physiologic stress from rapid caloric restriction and weight loss rather than a direct pharmacologic drug effect [24]. ## References [1] Wilding JPH et al. STEP 1. N Engl J Med. 2021;384:989-1002. https://www.nejm.org/doi/full/10.1056/NEJMoa2032183 [5] Lincoff AM et al. SELECT. N Engl J Med. 2023. https://www.nejm.org/doi/full/10.1056/NEJMoa2307563 [11] Wilding JPH et al. STEP 1 DXA substudy. J Endocr Soc. 2021. https://pmc.ncbi.nlm.nih.gov/articles/PMC8089287/ [12] Wilding JPH et al. STEP 1 adverse events. N Engl J Med. 2021. https://www.nejm.org/doi/full/10.1056/NEJMoa2032183 [13] Marso SP et al. SUSTAIN-6 retinopathy. N Engl J Med. 2016;375:1834-1844. https://www.nejm.org/doi/full/10.1056/NEJMoa1607141 [14] Davies M et al. STEP 2 safety. Lancet. 2021;397:971-984. https://www.thelancet.com/journals/lancet/article/PIIS0140-6736(21)00213-0/abstract [19] Perkovic V et al. FLOW. N Engl J Med. 2024;391:109-121. https://www.nejm.org/doi/10.1056/NEJMoa2403347 [24] Alopecia and Semaglutide. PMC11909624. 2025. https://pmc.ncbi.nlm.nih.gov/articles/PMC11909624/ --- Clinical trial record for one compound — semaglutide — read from the primary sources and indexed here. Not a prescriber, not a vendor.