A GP guide for obesity management in Dunedin, Otago & Southland
At Southern Weight Loss Dunedin, we work closely with GPs across Otago, Southland, Queenstown, Wanaka, Invercargill and Oamaru to deliver evidence-based obesity treatment.
With the availability of Mounjaro (tirzepatide) in New Zealand and established surgical options such as One Anastomosis Gastric Bypass (OAGB), primary care clinicians are increasingly asked:
Should my patient stay on medication long term, or consider metabolic surgery?
This guide provides a clinically practical, New Zealand-specific comparison to assist referral decisions.
Obesity treatment in New Zealand: two highly effective pathways
Obesity is a chronic, relapsing metabolic disease. Both pharmacotherapy and bariatric surgery now produce clinically meaningful weight loss and metabolic improvement. However, they differ fundamentally in durability, cost trajectory, and remission rates for obesity-related comorbidities.
At Southern Weight Loss (Mercy Hospital, Dunedin), we frequently see patients who:
- Have plateaued on GLP-1/GIP therapy
- Cannot tolerate escalation’
- Are concerned about long-term medication cost
- Seek definitive metabolic intervention
Mechanism of action
Tirzepatide (Mounjaro®)
Tirzepatide is a dual GIP and GLP-1 receptor agonist that:
- Enhances glucose-dependent insulin secretion
- Suppresses glucagon
- Slows gastric emptying
- Reduces appetite centrally
Weight loss occurs via hormonal appetite suppression.
Important: When the medication is stopped, the effect reverses. Appetite signalling returns and weight regain commonly occurs.
One Anastomosis Gastric Bypass (OAGB)
OAGB involves:
- Creation of a long gastric pouch
- Single gastrojejunostomy
- Restriction and controlled malabsorption
- Significant incretin and bile acid signalling changes
Unlike medication, OAGB produces sustained metabolic reprogramming independent of ongoing pharmacotherapy.
Tirzepatide dosing schedule (NZ clinical practice)
Typical escalation:
- 2.5 mg weekly × 4 weeks
- 5 mg weekly × 4 weeks
- 7.5 mg weekly × 4 weeks
- 10 mg weekly by month 4 (common maintenance)
- Up to 15 mg if required
Clinical Advice: Go Slow
For GPs managing patients in Dunedin and regional Otago:
- Escalate only if hunger control is inadequate
- Delay dose increases if nausea persists
- Prioritise protein intake and resistance training
- Consider maintaining at 5–10 mg if effective
Tirzepatide Is Intended as lifelong therapy
A critical counselling point:
Tirzepatide controls obesity physiology while administered. It does not cure obesity.
Clinical data shows:
- Weight regain occurs after cessation
- Glycaemic control deteriorates
- Appetite suppression reverses
For most patients, long-term or lifelong therapy is required to maintain benefit.
This has important financial and adherence implications for patients in the Southern region where treatment is currently self-funded.
Expected weight loss comparison
| Time | Tirzepatide TBWL | OAGB TBWL |
| 1 month | 3–4% | 8–10% |
| 4 months | 10–12% | 20–25% |
| 8 months | 15–18% | 30–35% |
| 12 months | 18–22% | 30–35% |
| 24 months | 20–23% | 35–40% |
Surgery produces faster, greater and more durable total body weight loss.
Resolution of Obesity-related comorbidities
| Condition | Tirzepatide | OAGB |
| Type 2 Diabetes | 50–60% remission or major improvement | 70–85% remission |
| Hypertension | 40–50% improve | 60–70% improve |
| Dyslipidaemia | ~50% improve | ~70% improve |
| Obstructive Sleep Apnoea | 30–50% improve | 60–80% improve |
| NAFLD | Significant improvement | High rates of histologic improvement |
For insulin-treated or long-standing diabetes, metabolic surgery remains superior.
Complication profile comparison
Tirzepatide
Common:
- Nausea (20–30%)
- Diarrhoea
- Reflux
- Constipation
Less common:
- Gallstones
- Rare pancreatitis
- Lean mass reduction
Advantages:
- No anaesthetic risk
- Fully reversible
- No micronutrient deficiency risk
One Anastomosis Gastric Bypass (Southern Weight Loss, Dunedin)
Early risks:
- Bleeding (1–2%)
- Leak (<1%)
- VTE (<1%)
- Mortality ~0.1–0.2% in experienced centres
Late risks:
- Bile reflux (1–3%)
- Iron/B12 deficiency
- Marginal ulcer
- Rare protein malnutrition
Requires lifelong supplementation and annual GP monitoring.
Updated cost comparison – New Zealand Context
Cumulative Cost Comparison
| Time Point | Tirzepatide Cumulative Cost | OAGB Cost |
| 1 month | $550 | $24,500 |
| 4 months | ~$2,339 | $24,500 |
| 8 months | ~$5,095 | $24,500 |
| 12 months | ~$7,851 | $24,500 |
| 24 months | ~$16,119 | $24,500 |
Cost Interpretation for GPs in Otago & Southland
- At 12 months: ~$8,000
- At 24 months: >$16,000
- At ~3.5 years: medication cost exceeds surgical package cost
- Higher doses (12.5–15 mg) increase cumulative expenditure
Given the need for lifelong continuation to prevent regain, long-term cost modelling is essential during shared decision-making.
Patients who may not tolerate Tirzepatide
Tirzepatide
Approximately 10–15% discontinue due to:
- Persistent nausea
- Severe reflux
- Injection aversion
- Financial burden
In these patients, bariatric surgery often provides a more sustainable long-term pathway.
One Anastomosis Gastric Bypass (Southern Weight Loss, Dunedin)
Early risks:
- Bleeding (1–2%)
- Leak (<1%)
- VTE (<1%)
- Mortality ~0.1–0.2% in experienced centres
Late risks:
- Bile reflux (1–3%)
- Iron/B12 deficiency
- Marginal ulcer
- Rare protein malnutrition
Requires lifelong supplementation and annual GP monitoring.
Which patients should GPs consider referring?
Early referral to Southern Weight Loss (Dunedin) may be appropriate for:
- BMI ≥35 with comorbidities
- Long-standing Type 2 Diabetes
- Insulin dependence
- Severe OSA
- Repeated medication failure
- Younger patients seeking durable metabolic remission
We accept GP referrals from across:
- Dunedin
- Otago
- Southland
- Queenstown
- Wanaka
- Invercargill
- Oamaru
Strategic Summary
| Factor | Tirzepatide | OAGB |
| Invasiveness | Non-surgical | Surgical |
| Weight Loss Magnitude | Moderate–High | High |
| Durability | Lifelong therapy required | Durable |
| Upfront Cost | Low | High |
| Long-Term Cost | Accumulative | Fixed |
| Diabetes Remission | Moderate | High |
Which patients should GPs consider referring?
For GPs managing obesity in Dunedin, Otago and Southland, both tirzepatide and One Anastomosis Gastric Bypass are powerful tools.
- Tirzepatide offers strong metabolic improvement with low procedural risk but requires lifelong administration and carries cumulative cost.
- OAGB provides greater and more durable weight loss with superior diabetes remission, at the cost of surgical risk and ongoing supplementation.
The optimal pathway depends on:
- BMI
- Diabetes severity
- Patient preference
- Tolerance of medication
- Financial considerations
- Long-term goals
At Southern Weight Loss, we work collaboratively with local GPs to individualise treatment and provide comprehensive pre- and post-operative support.
This article is intended for healthcare professionals. Patients should be assessed individually in consultation with a bariatric specialist.
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