GLP-1 side effect tracker

Why most GLP-1 side effects show up the way they do

GLP-1 receptor agonists (semaglutide, liraglutide, dulaglutide, exenatide) and the dual GIP/GLP-1 agonist tirzepatide work by stimulating receptors in the pancreas, brainstem, hypothalamus, and enteric nervous system. Three of those target sites explain about 80% of the side-effect profile: slowed gastric emptying (nausea, fullness, reflux), altered gut motility (constipation and diarrhea, often alternating), and central satiety signaling (decreased appetite, which cascades into fatigue and reduced hydration if you do not prioritize fluids).

In STEP 1, the week-68 semaglutide arm reported nausea in 43.9% of patients, diarrhea in 31.5%, vomiting in 24.8%, constipation in 24.2%, and “GI events leading to discontinuation” in 4.5%. SURMOUNT-1 tirzepatide 15 mg showed nausea in 29.3%, diarrhea 23.0%, constipation 17.1%, vomiting 12.9%. Most events were mild-to-moderate and transient, occurring during dose escalation rather than maintenance.

Pharmacokinetics — why day 3 is the worst

Semaglutide peaks in plasma roughly 72 hours after subcutaneous injection and has a half-life of ~1 week, which is why most patients feel side effects most sharply on days 2–4 post-injection and experience a “breakthrough appetite” window on days 6–7 as the level drifts down. Tirzepatide peaks slightly earlier (24–72 hours) with a similar 5-day half-life. The practical scheduling play: inject on an evening when day 3 will be your least-demanding day (many patients pick Friday evening so Monday is the rough day and the week builds forward from there). If you work Monday–Friday and have a commitment-heavy Thursday, moving your injection to Sunday evening often improves workweek tolerability materially. For liraglutide (Saxenda, daily), side effects cluster in the 4–8 hour window after each injection; morning dosing plus food typically works better than evening.

Severity grading your clinician will recognize

Obesity-medicine and oncology practice use the CTCAE (Common Terminology Criteria for Adverse Events) framework to standardize side-effect severity. The tracker’s 0–10 scale maps roughly to CTCAE grades:

• 0–2 (Grade 1, mild): asymptomatic or mild symptoms; no intervention needed.
• 3–5 (Grade 2, moderate): limiting some activities but not interfering with work/life; minimal supportive care.
• 6–8 (Grade 3, severe): limiting self-care; urgent outpatient evaluation warranted.
• 9–10 (Grade 4, life-threatening): ER-level evaluation, inability to tolerate oral intake, signs of dehydration or bleeding.

Red-flag symptoms that warrant a same-week call

• Vomiting ≥5/10 persistent for more than 48 hours: dehydration and electrolyte depletion risk. Ask about ondansetron and a dose hold.
• Diarrhea ≥7/10 or black/bloody stools: evaluate for other causes (C. difficile, ischemic colitis) and electrolyte panel.
• Severe abdominal pain radiating to the back: pancreatitis must be ruled out with lipase. Class-effect warning on all GLP-1s.
• Right upper quadrant pain, fever, jaundice: cholecystitis and cholelithiasis are modestly more common during rapid weight loss. Needs urgent eval.
• New visual changes on diabetes dose: diabetic retinopathy can worsen with rapid A1C improvement. Ophthalmology follow-up.
• Lump or fullness in the neck, hoarseness, difficulty swallowing: extremely rare, but the medullary thyroid carcinoma boxed warning justifies prompt evaluation.
• Signs of severe hypoglycemia: only a risk in patients co-prescribed insulin or sulfonylureas. Adjust those first, not the GLP-1.

What to actually do for mild-to-moderate symptoms

Most nausea, constipation, and fatigue resolves within 2–4 weeks at a stable dose. Practical interventions that are well-supported:

• Nausea: eat small, frequent, lower-fat meals; cold or room-temperature foods tolerate better than hot; avoid lying flat within 2 hours of eating; ginger or peppermint can help; prescription ondansetron 4–8 mg as needed is a standard PRN for moderate cases.
• Constipation: fluid ≥80 oz/day, fiber 25–35 g/day (psyllium or food), daily walking. Magnesium citrate 400 mg at bedtime or Miralax daily are first-line OTC.
• Reflux: avoid eating within 3 hours of bed, elevate head of bed 6 inches, famotidine 20 mg as needed. Escalate to a PPI if persistent.
• Fatigue: usually signals inadequate intake or electrolyte depletion. Enforce 20–30 g protein per meal (shake if necessary), 2–3 L fluid/day with electrolytes, and verify ferritin and vitamin D on labs.
• Hair shedding: telogen effluvium begins 2–4 months into rapid loss and resolves 3–6 months after loss stabilizes. Protein and adequate iron help. See the hair loss tool.
• Injection-site reactions: rotate sites (abdomen, thigh, upper arm), use a fresh needle, and let the pen warm 5 minutes out of the fridge before injecting. Swelling >1 inch or warmth/redness 48+ hours after injection needs evaluation.

The class-effect pattern by drug

Not every GLP-1 produces the same side-effect signature. The STEP and SURMOUNT programs let us compare head-to-head:

• Semaglutide 2.4 mg (STEP 1, N=1,961): nausea 43.9%, diarrhea 31.5%, vomiting 24.8%, constipation 24.2%. Discontinuation for GI events 4.5%. Most events mild-moderate and clustered at escalation steps.
• Tirzepatide 15 mg (SURMOUNT-1, N=2,539): nausea 29.3%, diarrhea 23.0%, constipation 17.1%, vomiting 12.9%. Discontinuation for GI events 4.3%. The dual GIP effect appears to slightly moderate the nausea signal compared with pure GLP-1 agonism.
• Liraglutide 3.0 mg (SCALE): nausea 40.2%, vomiting 16.3%, diarrhea 20.9%. Daily injection; side effects cluster in the hours after each injection rather than the weekly pharmacokinetic peak.
• Dulaglutide 1.5–4.5 mg (AWARD-11): nausea 14–25%, generally better tolerated than semaglutide at comparable glycemic effect.

If you cannot tolerate one agent at titration, switching within the class before giving up is a reasonable next step — approximately 30% of semaglutide intolerance is resolved by a switch to tirzepatide with a fresh low-dose start.

When to hold vs. stop

Holding means staying at the current dose (or stepping back to the prior dose) for 4 additional weeks to let the gut adapt. Stopping means discontinuing the drug. The threshold:

• Hold: persistent moderate symptoms (3–5/10) during a titration step; dose escalation scheduled within 2 weeks; GI symptoms interfering with hydration.
• Step down: severe symptoms (≥6/10) after recently escalating; return to prior dose for 4 weeks, then re-attempt escalation.
• Discontinue and do not rechallenge: suspected pancreatitis, gallstone-related severe pain requiring intervention, severe hypersensitivity.
• Continue and manage symptomatically: Grade 1 symptoms (≤2/10), mild fatigue, mild transient nausea in the 24–48 hours after injection.

Discontinuation rates in the trials and in the real world

STEP 1 showed overall discontinuation at 6.9% in the semaglutide arm vs 3.1% in placebo; GI-specific discontinuation was 4.5% vs 0.8%. SURMOUNT-1 showed ~4.3% overall discontinuation in the tirzepatide 15 mg arm for GI events. These are low numbers — and they reflect highly supervised trial populations with proactive side-effect management. Real-world data is materially worse: a Blue Cross 2024 analysis of commercial Wegovy/Zepbound starts found <25% of patients remained on therapy at 12 months. Drop-off concentrates at two points: weeks 3–6 of the first titration, and the 1.0 → 1.7 mg or 7.5 → 10 mg escalation steps. Most of the real-world gap between trial efficacy and population efficacy is discontinuation-driven, not biology-driven. Aggressive side-effect management in the first 12 weeks is the single highest-leverage intervention for outcomes.

Bringing this to your prescriber

Print this tracker or screenshot the severity bar chart. What obesity-medicine clinicians actually want to know at your follow-up visit: (1) the symptom list and severity, (2) the timing relative to injection day, (3) whether the symptoms started with the current dose or a prior dose, (4) what you have tried for relief and whether it worked, (5) weight change and waist change since last visit. The chart above answers #1 and #2. Keep a brief text log for the rest.

FAQ

My nausea is terrible two days after injection and fine the rest of the week. Normal?

Very normal. Semaglutide peaks in plasma at day 3–4 after injection. Schedule the injection for a day when day 3 is your least-busy (many patients choose Friday or Saturday). Consider pre-medicating with ondansetron on peak days if tolerated.

How long does nausea last overall?

In the STEP trials, the majority of nausea events resolved within 48 hours of each dose and within 4 weeks of each dose escalation. By maintenance dose on 2.4 mg semaglutide or 15 mg tirzepatide, persistent nausea affects <10% of patients.

I’m losing hair at month 4 — should I stop?

Almost never. GLP-1-associated hair shedding is telogen effluvium, a temporary shift of follicles into the rest phase in response to the metabolic stress of rapid loss. It resolves within 3–6 months after weight stabilizes. Protein, iron, and zinc sufficiency help. See the hair-loss risk tool.

What if I vomit right after injecting?

Semaglutide and tirzepatide are absorbed subcutaneously, not orally, so vomiting does not affect the dose you received. Manage the vomiting; do not re-inject.

Does the severity usually get worse at higher doses?

Mostly the opposite — side effects cluster around dose escalations rather than dose levels. At any stable dose, 80–90% of patients adapt within 4 weeks. The 1.0 → 1.7 mg semaglutide step and the 7.5 → 10 mg tirzepatide step tend to be the hardest in real-world practice.

Related tools

• Dosage schedule generator — check what step you are on
• Hair loss risk tool
• Muscle loss risk
• Diet on GLP-1 — nausea-friendly protein strategy