GLP-1 Side Effects: What to Expect in the First Month of Treatment

Most Common Side Effects in Month One

1. Nausea — The Most Reported Side Effect

Nausea is the single most common side effect of semaglutide. In STEP 1, 44.2% of semaglutide patients reported nausea at some point during treatment, compared to 16.0% in the placebo group.

The good news: nausea is most intense during the dose escalation phase — when your dose is being increased — and tends to diminish significantly after 4–8 weeks. For most patients, it is manageable rather than severe.

Why it happens: GLP-1 receptors in the brainstem regulate nausea and vomiting. As the medication slows gastric emptying and activates these receptors, the brain initially interprets this as a signal of fullness or mild digestive stress.

How to manage it:

• Eat smaller, more frequent meals
• Avoid high-fat, greasy, or strongly spiced foods — these are harder to digest with slowed gastric emptying
• Eat slowly and chew thoroughly
• Stay upright for 30–60 minutes after eating
• Avoid lying down immediately after meals
• Ginger tea, plain crackers, or cold water can help settle the stomach
• If nausea is severe, speak with your prescribing doctor — antiemetics can be prescribed

2. Vomiting

Vomiting was reported by 24.8% of semaglutide patients in STEP 1, versus 6.8% on placebo. As with nausea, it peaks during dose increases and typically subsides.

When to take it seriously: Persistent vomiting (more than 2–3 episodes per day lasting more than a day or two) warrants medical review. Dehydration is a risk, and your doctor may adjust your dose escalation schedule.

3. Diarrhoea

Approximately 29.7% of semaglutide patients in STEP 1 experienced diarrhoea, compared to 15.9% on placebo. Again, this is most common early in treatment and during dose increases.

Management: Stay well hydrated. Avoid high-fibre foods immediately after injection day if sensitivity is high. Most cases resolve within a few days.

4. Constipation

Perhaps counterintuitively, constipation is also common — 24.2% of semaglutide patients in STEP 1 versus 10.8% on placebo. Slowed gastric motility can reduce bowel frequency.

Management: Increase fluid intake significantly. Ensure adequate dietary fibre from vegetables and legumes. Light physical activity — even a 20-minute walk — can help stimulate motility. If constipation is severe or persists, contact your doctor.

5. Injection Site Reactions

Mild redness, itching, or localised swelling at the injection site was reported in approximately 3.1% of patients in STEP 1. This is generally mild and transient.

Management: Rotate injection sites each week (abdomen, upper thigh, upper arm). Allow the pen to come to room temperature before injecting. Do not inject into skin that is bruised, irritated, or damaged.

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Less Common But Important Side Effects

Headache and Fatigue

Headaches affect some patients in the first few weeks, often linked to reduced caloric intake or mild dehydration from GI symptoms. Ensuring adequate hydration and maintaining caloric intake above 1,200 kcal/day typically helps.

Dizziness

Some patients experience mild dizziness, particularly when standing up quickly. This can be related to lower blood sugar levels (if you're also on diabetes medication) or reduced food intake. Report persistent dizziness to your doctor.

Reduced Appetite and Early Satiety

This is intentional — it's how the medication works. However, some patients eat too little as a result, particularly in the first month. It's important to continue eating balanced, nutritious meals even when your appetite is suppressed. Malnutrition can lead to muscle loss, fatigue, and micronutrient deficiencies.

A qualified healthcare provider will typically recommend:

• Prioritising protein intake (≥1.2 g/kg ideal body weight per day) to preserve lean muscle mass
• Continuing vitamin supplementation if recommended
• Tracking food intake to ensure nutritional adequacy

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When to Seek Urgent Medical Attention

The following symptoms require immediate medical evaluation. Stop medication and contact your doctor or go to an emergency department if you experience:

Pancreatitis (inflammation of the pancreas):

• Severe, persistent abdominal pain, often radiating to the back
• Nausea and vomiting that does not settle
• Note: The absolute risk is low. STEP 1 reported 5 cases of pancreatitis in the semaglutide group versus 1 in placebo — rare, but requires prompt attention

Gallbladder problems:

• Sudden pain in the upper right abdomen
• STEP 1 reported a higher rate of cholelithiasis (gallstones) in the semaglutide group (1.6%) vs placebo (0.7%) — rapid weight loss itself is a known risk factor for gallstone formation

Severe allergic reactions:

• Widespread rash, difficulty breathing, facial swelling
• Rare, but requires emergency care

Heart rate changes:

• Semaglutide can increase resting heart rate by a mean of 2–3 bpm. Patients with pre-existing cardiac arrhythmias should discuss this with their cardiologist before starting

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Who Should NOT Use GLP-1 Medications

Semaglutide and other GLP-1 receptor agonists are contraindicated in:

• Personal or family history of medullary thyroid carcinoma (MTC) or Multiple Endocrine Neoplasia type 2 (MEN2) — based on animal study findings (rodent thyroid tumours at high doses; relevance to humans is uncertain, but it remains a formal contraindication)
• Pregnancy or planned pregnancy — GLP-1 medications are not recommended during pregnancy; use effective contraception during treatment
• Active pancreatitis or history of pancreatitis
• Severe renal or hepatic impairment — dose adjustment or avoidance may be required; your doctor will assess

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Managing the First Month: A Practical Summary

The first month of GLP-1 treatment is the most challenging for most patients. The dose is being titrated upward, your body is adapting to a new hormonal signal, and GI side effects are at their peak.

What helps most patients get through it:

| Strategy | Why It Works | |---|---| | Small, frequent meals | Reduces gastric overload with slowed emptying | | Low-fat food choices | Fat slows digestion further — compounding GI symptoms | | Hydration (2–2.5L/day) | Counters nausea, constipation, dizziness | | Adequate protein intake | Preserves muscle during caloric reduction | | Injection rotation | Prevents site reactions | | Open communication with your doctor | Dose adjustments are available if side effects are unmanageable |

The majority of patients who experience GI side effects in month one find them substantially improved by weeks 6–8. Persistence through the early phase is associated with better long-term outcomes.

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Starting Treatment in Singapore

GLP-1 medications for weight management are available in Singapore via licensed physicians on a prescription basis. A proper medical assessment — including review of your medical history, current medications, and contraindications — is required before treatment begins.

Noah is a licensed telehealth provider in Singapore offering physician-supervised GLP-1 treatment programmes. Assessment is conducted by qualified doctors. Treatment includes ongoing clinical support throughout your programme, including guidance on managing side effects.

If you're ready to understand whether GLP-1 treatment is right for you, start your assessment at ofnoah.sg.

This article is for informational purposes only and does not constitute medical advice. GLP-1 medications are available by prescription only in Singapore. Consult a licensed physician before starting any new medication.