Journal · Updated 2026-08-11

GLP-1 Side Effects A to Z: The Complete Tirzepatide and Semaglutide Reference

By the GLP1ProviderFinder Research Desk · Medically reviewed by Dr. A. Goher, MD · Last reviewed 2026-08-11 · How we verify

How to read this reference

Frequencies quoted here come from the FDA-approved labels for Zepbound and Mounjaro (tirzepatide) and Wegovy, Ozempic, and Rybelsus (semaglutide), and from the pivotal trials behind them — SURMOUNT for tirzepatide, STEP for semaglutide. Compounded versions carry the same active molecules and, in practice, the same side-effect profile, with the added variables of formulation and dosing accuracy that our pharmacy-verification guide exists to manage. Where tirzepatide and semaglutide differ meaningfully, each entry says so; where an entry has its own full article on this site, the link is the better read.

A — appetite suppression and early satiety

The intended effect, listed here because its intensity surprises people: both molecules slow gastric emptying and act on appetite circuits, and "forgetting to eat" is a commonly reported experience at higher doses. The management issue is nutritional — protein first, hydration always — covered in the protein guide and the muscle-preservation article. Appetite that collapses to the point of inability to maintain fluids is not a feature; that belongs on the call-your-prescriber list.

B — belching, sulfur burps, and bloating

Eructation is label-listed for both molecules, and the notorious sulfur-smelling variant has its own physiology — slowed emptying gives gut bacteria longer to work on food. Reported by a meaningful minority of users in trials (eructation ran roughly five to nine percent across the obesity-dose programs versus about one percent on placebo). Management, triggers, and when it signals something more are in the sulfur burps and reflux guide. B is also for birth control: tirzepatide specifically can reduce oral contraceptive absorption during initiation and after each dose escalation — the label advises a backup barrier method for four weeks after starting and after each step up. The birth-control interaction guide covers the mechanics; this is a tirzepatide-specific caution that semaglutide's label does not carry.

C — constipation

One of the most persistent complaints in both programs: roughly one in six to one in four participants at obesity doses in the trials, versus single digits on placebo, and unlike nausea it does not always fade with time because the mechanism — slowed transit — persists by design. Fluids, fiber, magnesium, movement, and the escalation path are covered in the constipation relief guide. No bowel movement for several days accompanied by pain, vomiting, or distension is an ileus concern, not a fiber problem — that is the emergency column.

D — diarrhea, dehydration, and dizziness

Diarrhea is the mirror complaint to constipation — roughly one in five at higher doses in the trials, often alternating with it. Its real danger is downstream: dehydration, which is the common pathway to the kidney-injury reports on both labels and the usual explanation for dizziness beyond simple under-eating. The dehydration and electrolytes guide is the practical read; persistent inability to keep fluids down is a same-day clinical call.

E — energy, fatigue, and eye findings

Fatigue is label-listed for tirzepatide (around six percent in the obesity trials) and widely reported for both molecules, usually tracking calorie deficit and titration weeks — the fatigue guide separates the benign from the flag-raising. The eye entries are two and specific. First, in people with type 2 diabetes and existing retinopathy, rapid blood-sugar improvement on semaglutide was associated with early retinopathy worsening in the SUSTAIN-6 trial — a known phenomenon of fast glycemic correction; diabetics with eye disease should be under ophthalmology follow-up when starting. Second, regulators have examined reports of non-arteritic anterior ischemic optic neuropathy (NAION), a rare optic-nerve event, in semaglutide users; the European Medicines Agency concluded in 2025 that it is a possible side effect occurring very rarely — on the order of one in ten thousand or fewer. Any sudden vision loss or new visual-field defect on either molecule is an immediate evaluation, full stop.

F — food aversion and taste changes

Dysgeusia (altered taste) appears in a small percentage of trial participants, and aversions — most famously to meat, fried food, and alcohol — are commonly reported. Mostly a curiosity; occasionally the wedge that makes protein targets hard, which loops back to the muscle-loss reading above.

G — gallbladder disease and gastroparesis

Two of the serious entries. Gallbladder: rapid weight loss itself raises gallstone risk, and both labels report it — cholelithiasis in roughly one to two percent of obesity-dose participants and cholecystitis in a fraction of a percent, higher than placebo. Right-upper-abdominal pain, especially after fatty meals, with fever or yellowing, is the presentation. The gallbladder and pancreatitis article carries the full numbers. Gastroparesis: slowed gastric emptying is the mechanism, not a malfunction — but a small number of users experience severe, persistent delayed emptying with intractable vomiting, and post-marketing reports of ileus (intestinal blockage-like slowdown) appear on the semaglutide labels. Persistent vomiting hours after eating is the sign that the intended effect has overshot.

H — hair loss, headaches, and heartburn

Hair loss is real, usually telogen effluvium from rapid weight loss rather than a direct drug effect: about five percent of tirzepatide participants versus one percent on placebo in the obesity trials, and roughly three percent versus one for semaglutide — timing, regrowth expectations, and the nutrient checklist are in the hair-loss guide. Headaches mostly track dehydration, under-eating, and caffeine changes rather than the molecule — the headache article audits the evidence. Heartburn: GERD and dyspepsia are label-listed for both; slowed emptying keeps stomach contents around longer. The reflux guide covers positioning, timing, and when reflux plus vomiting stops being routine.

I — injection-site reactions

Redness, itching, or a small bruise at the site — a few percent in trials, usually trivial and technique-responsive. Rotation, needle habits, and what an infected site looks like are in the injection-site guide, with the mechanics in the technique article. True allergic reactions are separate and rare: facial or throat swelling, hives, or breathing difficulty after a dose is anaphylaxis territory — emergency care, not observation.

J, K — joints and kidneys

Joint aches appear in reports though not prominently in labels; substantial weight loss usually helps joints on net. Kidneys are the label item: acute kidney injury reports on both molecules cluster around volume depletion — vomiting and diarrhea leading to dehydration in people who are often also on blood-pressure medications. People with existing kidney disease warrant closer monitoring, and hydration during GI weeks is genuinely protective. Notably, in people with type 2 diabetes and chronic kidney disease, semaglutide demonstrated kidney protection in the FLOW trial — the same molecule, a different context, and the basis of one of the insurance doors.

L, M — low blood sugar and muscle

Hypoglycemia on these molecules alone is uncommon in non-diabetics; the risk concentrates in combination with insulin or sulfonylureas, where the labels advise dose adjustment of those drugs — the insulin and sulfonylurea interaction guide covers it. Shakiness, sweating, and confusion that resolve with sugar is the pattern. Muscle: a real fraction of rapid weight loss is lean mass; the trial body-composition data and the protein-plus-resistance playbook are in the muscle-loss article.

N — nausea

The signature side effect: roughly a quarter to a third of participants at obesity doses (semaglutide's trials ran modestly higher than tirzepatide's), overwhelmingly during titration weeks and after dose increases, fading at stable dose for most. The week-by-week shape is charted in the side-effect timeline and the countermeasures in the nausea remedies guide. Nausea that prevents fluid intake for more than a day is the escalation point.

P — pancreatitis and pregnancy

Pancreatitis is the serious entry both labels lead with: uncommon in trials (a fraction of a percent) but consequential — severe, persistent upper-abdominal pain, often radiating to the back, with or without vomiting, means stop the medication and seek care the same day. History of pancreatitis is a start-of-care disclosure, and the deep-dive carries the trial numbers. Pregnancy: neither molecule is for use in pregnancy; weight loss itself is contraindicated then. Semaglutide's long half-life drives the label advice to discontinue at least two months before a planned conception; tirzepatide adds the oral-contraceptive caution above. The women's guide and breastfeeding article cover the adjacent decisions.

R, S — reflux, surgery, and the suicidality question

Reflux is covered under H. Surgery: because these molecules delay gastric emptying, anesthesia societies advise telling your surgical and anesthesia team well in advance — retained stomach contents raise aspiration risk under sedation, and hold-the-dose protocols exist; the pre-surgery guide is the checklist. Suicidality: after case reports, FDA conducted a preliminary review announced in January 2024 and found no evidence of a causal link between GLP-1s and suicidal thoughts, a conclusion European regulators broadly matched — labels nonetheless advise monitoring mood changes, and any emergent suicidal ideation on treatment is a same-day clinical contact regardless of attribution. Our antidepressant interaction guide sits adjacent.

T — thyroid: the boxed warning

The most prominent warning on every label in the class: in rodent studies, GLP-1 receptor agonists caused dose-dependent thyroid C-cell tumors; whether this translates to humans is unknown, and it has not been established in human data. The labels therefore contraindicate both molecules in anyone with a personal or family history of medullary thyroid carcinoma (MTC) or Multiple Endocrine Neoplasia syndrome type 2 (MEN2) — a screening question every legitimate intake asks, and one reason an intake that skips history questions is a red flag our intake guide flags. A new neck lump, persistent hoarseness, or trouble swallowing on treatment warrants evaluation.

V, W — vomiting and weight regain

Vomiting ran high single digits to the low teens at obesity doses in the trials — episodic vomiting during titration is common; persistent or projectile vomiting, vomiting with severe pain, or inability to keep fluids down crosses into the serious column (dehydration, kidney risk, possible pancreatitis or obstruction). Weight regain after stopping is the best-documented "side effect" of discontinuation: the withdrawal arms of the trials showed most lost weight returning over the following year — the stopping-and-regain article reads that evidence closely, because it reshapes the cost math this entire site exists to compute.

The stop-and-call list

Same-day clinical contact or emergency care, no waiting: severe persistent abdominal pain, especially radiating to the back (pancreatitis); right-upper-quadrant pain with fever or jaundice (gallbladder); persistent vomiting or inability to hold fluids beyond a day (dehydration, obstruction); signs of allergic reaction — swelling of face or throat, hives, difficulty breathing; sudden vision change of any kind; symptoms of low blood sugar that recur, if you also take insulin or a sulfonylurea; a new neck mass or persistent hoarseness; new or worsening thoughts of self-harm. And a reporting note that applies doubly to compounded products: adverse events can and should be reported to FDA MedWatch — reports are exactly how the safety record of the compounded lane gets written, a theme our gray-market analysis returns to.

Questions people ask

What are the most common side effects of tirzepatide and semaglutide?

Gastrointestinal, by a wide margin: nausea (roughly a quarter to a third of trial participants at obesity doses), constipation and diarrhea (each roughly one in five to six), vomiting, reflux, and belching — dose-dependent, worst during titration, and fading at stable doses for most people. Serious label-listed risks (pancreatitis, gallbladder disease, kidney injury from dehydration) are uncommon but require knowing the warning signs.

Which GLP-1 side effects mean I should stop and call a doctor?

Severe persistent abdominal pain (especially radiating to the back), right-upper-belly pain with fever or jaundice, vomiting you can't keep fluids through for a day, face/throat swelling or trouble breathing, sudden vision changes, recurrent low-blood-sugar symptoms if you take insulin or a sulfonylurea, a new neck lump or persistent hoarseness, or new thoughts of self-harm. Same-day contact — not a wait-and-see list.

Do compounded tirzepatide and semaglutide have different side effects than brand?

Same active molecules, same expected profile. The compounded lane adds formulation and dosing-accuracy variables — vial-and-syringe dosing errors are documented in FDA case reports — which is why pharmacy verification and label-reading matter there in a way they don't with a factory pen.

Is the thyroid cancer warning on GLP-1s based on human cases?

The boxed warning comes from rodent studies showing thyroid C-cell tumors; human relevance is unknown and not established. The practical consequence is absolute, though: anyone with a personal or family history of medullary thyroid carcinoma or MEN2 should not take these medications, and every legitimate intake screens for it.

Did the FDA find that GLP-1s cause suicidal thoughts?

No — FDA's preliminary review, announced January 2024, found no evidence of a causal link, and European regulators reached broadly the same conclusion. Labels still advise monitoring mood, and any emergent suicidal ideation during treatment warrants same-day clinical contact regardless of cause.

This article is pricing research, not medical advice. Verify figures at the provider's checkout. Nothing here is medical advice.