GLP-1 (glucagon-like peptide-1) receptor agonists are a class of drugs used primarily to manage type 2 diabetes and obesity.
These drugs work by mimicking the action of the natural hormone GLP-1, which is involved in glucose regulation and appetite control.
Here’s how they work:
1. Increase Insulin Secretion
• Action: GLP-1 drugs enhance insulin release from the pancreas when blood glucose levels are high. This helps lower blood sugar levels after meals.
• Mechanism: They bind to GLP-1 receptors on pancreatic beta cells, triggering insulin secretion in a glucose-dependent manner. This means insulin is released only when blood sugar levels are elevated, reducing the risk of hypoglycemia (low blood sugar).
2. Suppress Glucagon Secretion
• Action: GLP-1 drugs reduce the secretion of glucagon, a hormone produced by the pancreas that raises blood glucose levels.
• Mechanism: By inhibiting glucagon release, these drugs prevent the liver from producing excess glucose, particularly during fasting, thereby helping to stabilize blood glucose levels.
3. Slow Gastric Emptying
• Action: GLP-1 drugs slow down the movement of food through the stomach (gastric emptying), which helps improve satiety and control post-meal blood sugar spikes.
• Mechanism: By slowing gastric emptying, the drugs make the stomach feel fuller for longer, which can aid in weight loss and reduce appetite.
4. Increase Satiety (Appetite Suppression)
• Action: GLP-1 receptor agonists also act on the brain to reduce appetite and promote a feeling of fullness.
• Mechanism: They interact with centers in the brain that regulate hunger, particularly in the hypothalamus, reducing the urge to eat and promoting weight loss.
5. Improve Beta-Cell Function
• Action: Some evidence suggests that GLP-1 drugs can improve the function of insulin-producing beta cells in the pancreas, which can be particularly beneficial for people with diabetes who have impaired beta-cell function.
• Mechanism: GLP-1 receptor activation may promote beta-cell growth and survival, improving long-term glucose control.
Summary of Effects:
• Blood Sugar Control: By stimulating insulin release and inhibiting glucagon secretion, GLP-1 drugs help control blood glucose levels, especially after meals.
• Weight Loss: They reduce appetite and slow gastric emptying, leading to weight loss, making them beneficial for patients with both obesity and type 2 diabetes.
Common GLP-1 Drugs:
• Semaglutide (Ozempic, Wegovy, Rybelsus)
• Tirzepatide (Mounjaro)
• Liraglutide (Saxenda)
• Dulaglutide (Trulicity)
• Exenatide (Byetta)
Long-Term Benefits of GLP-1 Agonists
• Improved glycemic control in type 2 diabetes
• Sustained weight loss
• Reduced cardiovascular risk (e.g., stroke, heart attack in high-risk patients)
• Possible protection against progression of kidney disease
Medication Comparison Table
| | | | | | | |
| | | | | | | Nausea, vomiting, diarrhea, abdominal pain, headache |
| | | | | | Not subsidized by PBS; higher out-of-pocket cost | |
| | | | | | | Nausea, vomiting, diarrhea, constipation, potential pancreatitis |
| | | | | | ~$130/month (private script) | Nausea, vomiting, diarrhea, abdominal pain |
Potential Long-Term Risks of GLP-1 Agonists
🔹 1. Gastrointestinal Effects
• Very common and usually dose dependent. May persist or return with higher doses.
• Nausea, vomiting, diarrhea, constipation
• Long-term GI issues can lead to malnutrition or dehydration in some cases
Tip: Slow dose titration can reduce symptoms
🔹 2. Pancreatitis (Acute and Chronic)
• Rare, but serious. Can present as persistent upper abdominal pain radiating to the back.
• Incidence is low, but caution is warranted in those with:
o History of pancreatitis
o Gallbladder disease
No confirmed increase in pancreatic cancer in long-term trials, but under continued review.
🔹 3. Gallbladder Disease
• Increased risk of gallstones and cholecystitis
• Possibly related to rapid weight loss
🔹 4. Muscle Loss (Sarcopenia)
• Significant weight loss from GLP-1s can include lean muscle mass, especially in older adults or those not exercising
• Risk of frailty or sarcopenic obesity over time
• Strength training and adequate protein intake are key for prevention
🔹 5. Thyroid C-cell Tumours (in rodents)
• Seen in rat studies (medullary thyroid carcinoma – MTC), not confirmed in humans
• Contraindicated in patients with:
o Personal/family history of MTC
o MEN2 (Multiple Endocrine Neoplasia syndrome type 2)
• No proven human cases, but patients should report neck lumps, hoarseness, or difficulty swallowing
🔹 6. Mental Health Effects (Emerging)
• Some reports (though not clearly proven) of:
o Low mood, anxiety, or disordered eating
o Potential body image issues tied to rapid weight loss
🔹 7. Hypoglycaemia
• Rare when used alone
• Risk (in diabetics) increases if combined with:
o Sulfonylureas (e.g., gliclazide)
o Insulin
🔹 8. Kidney Injury (Secondary)
• Usually from dehydration due to severe vomiting/diarrhea
• Caution in patients with existing chronic kidney disease (CKD) or volume depletion
🔹 9. (Possible) Increased Risk of Non-arteritic Anterior Ischemic Optic Neuropathy (NAION)
Non-Arteritic Anterior Ischemic Optic Neuropathy (NAION):
• is a condition characterized by sudden, painless vision loss in one eye due to reduced blood flow to the optic nerve, leading to optic disc swelling and potential optic atrophy.
• Vision loss is often irreversible, with ~40-43% of patients showing some visual acuity improvement.
• Risk of NAION in the second eye is ~15% over 5 years; recurrence in the same eye is rare (~6.4%).
Who Gets NAION?
• NAION is the most common acute optic neuropathy in people over 50, with an incidence of 2-10 per 100,000 annually.
• In Australia, NAION risk is higher in individuals with type 2 diabetes (5.3% prevalence) and obesity (67% of adults), as these are common comorbidities.
• Aboriginal and Torres Strait Islander populations may have elevated risk due to higher rates of diabetes and hypertension.
Risk factors include:
• Age: Most common in individuals over 50, with risk increasing with age due to vascular changes.
• Sex: Slightly more common in men (54.5% in some studies).
• Anatomical Predisposition: A small cup-to-disc ratio (“disc-at-risk”) in the optic nerve head, characterized by crowded optic nerve fibers, is the strongest risk factor. This anatomical trait predisposes individuals to microvascular occlusion.
• Cardiovascular Risk Factors:
o Hypertension: Present in many NAION cases, contributing to vascular insufficiency.
o Diabetes Mellitus (Type 2): Increases risk due to microvascular damage; prevalent in NAION patients.
o Hyperlipidemia: High cholesterol promotes atherosclerosis, reducing optic nerve blood flow.
o Coronary Artery Disease: Shares vascular pathology with NAION.
o Obstructive Sleep Apnea (OSA): Associated with nocturnal hypoxia and vascular dysregulation, increasing NAION risk.
o Obesity: Linked to higher NAION incidence, possibly due to associated comorbidities like diabetes and OSA.
o Smoking: May elevate risk by worsening vascular health, though evidence is less definitive.
o Systemic Hypoperfusion: Conditions like nocturnal hypotension or hemorrhagic shock (e.g., from gastric bleeding) can trigger NAION.
• Medications: Certain drugs (e.g., phosphodiesterase type 5 inhibitors like sildenafil, amiodarone) have been associated with NAION, though causality is debated.
• Surgical Procedures: General surgeries, including cataract surgery, may precipitate NAION due to hypotension or anaemia (incidence: 0.002-0.1%).
Is There an Increased Risk of NAION with GLP-1 Agonist Drugs?
• Recent studies have investigated a potential link with NAION, but evidence is conflicting and causality is not established.
• Evidence is mixed.
• Some studies (e.g., Hathaway, Danish-Norwegian) suggest a 2-7-fold increased NAION risk with semaglutide (Ozempic, Wegovy, Rybelsus) and possibly tirzepatide (Mounjaro), particularly in diabetes or obesity patients.
• However, larger studies and meta-analyses find no significant risk.
• The absolute risk is low, and causality is unproven.
• NAION risk factors (diabetes, obesity, hypertension) overlap with GLP-1 RA indications, complicating causality.
• No clear mechanism links GLP-1 RAs to NAION.
• Hypotheses include rapid glycemic control causing retinal hypoxia or vascular changes, but these are unproven.
• No TGA warnings exist in Australia as of June 2025.
• Patients with pre-existing NAION, monocular vision, or low vision may need caution with GLP-1 RAs until further data clarifies risk.
