On Ozempic Before a Knee or Hip Replacement? What Actually Changes

Five years ago this question did not come up. It now comes up most weeks.

A patient arrives at a consultation about an arthritic knee or hip, mentions they are on Ozempic, Wegovy or Mounjaro, and asks some version of the same three things: do I stop it before surgery, does it make the operation safer, and might it mean I do not need the operation at all?

There are reasonable answers to all three, and the first one matters most, because getting it wrong can cost you your surgery date.

1. No, you do not stop it — but the fasting changes

This is the one to get right, and it runs against most people’s instinct.

The 2025 Australian clinical practice recommendations — produced jointly by the Australian Diabetes Society, the Australia and New Zealand College of Anaesthetists, the Gastroenterological Society of Australia and NACOS — advise continuing GLP-1 medications through the period around a procedure rather than ceasing them. Stopping has no clear evidence of benefit, and it risks destabilising blood sugar and interrupting weight management for no gain.

What does change is how you fast.

These medications work partly by slowing how quickly the stomach empties. That is useful when the goal is feeling full for longer. It is a problem under anaesthetic, because food can still be sitting in the stomach after the standard overnight fast, and stomach contents that reach the lungs — pulmonary aspiration — is a rare but serious complication.

So the guidance asks for something more than the usual:

• Clear fluids only for the 24 hours before surgery, starting from when you wake the day before. There is no limit on how much you drink during that period.
• Then the standard fasting instructions on top of that — clear fluids until six hours before, water only from six hours, nothing at all for the final two.

If you turn up not having done the clear fluid day, your anaesthetist has options. They can scan your stomach with ultrasound, use medication to help it empty, change the anaesthetic technique, or defer the operation. That last one is the reason this is worth knowing weeks in advance rather than the night before.

Tell your surgeon and your anaesthetist that you are on one. Say the name and the dose. It is not a confession, and it is not a judgement — it is a piece of information that changes the plan.

And follow the instructions your own hospital and anaesthetist give you. Protocols vary a little between sites, and theirs override anything written here.

2. Being on one before surgery appears to lower complication risk

This is newer, and it is genuinely encouraging.

A team at Yale published a study in the Journal of Arthroplasty in January 2026 looking at patients with type II diabetes having a total knee replacement, grouped by how long they had been taking semaglutide beforehand — under a month, one to two months, two to three, three to six, six to twelve.

The pattern was a dose of time rather than a threshold:

• Under a month was associated with fewer minor complications — wound problems, bleeding, kidney issues, pneumonia.
• Two to three months or longer was associated with fewer serious ones — deep infection, sepsis, blood clots, cardiac events.

The authors’ practical conclusion was that around three months of exposure before surgery is where the meaningful protection sits.

There is a real caveat and it is worth stating plainly: this was a study of patients with diabetes. Whether the same benefit applies to someone taking a GLP-1 solely for weight loss has not been demonstrated. It is plausible, and it is not proven.

What it does support is the timing point. If you are on one of these medications and a replacement is on the horizon, a two to three month run-up is better than a two-week one. That is a scheduling conversation worth having early.

3. It may delay the replacement — but read the numbers carefully

The most eye-catching finding of the year came out on 2 June 2026 in Regional Anesthesia & Pain Medicine.

Researchers used a large international database to follow adults with knee osteoarthritis, matching those on GLP-1 medications against similar patients who were not. Among people taking semaglutide or tirzepatide for three years, roughly 1.8 per cent had gone on to a knee replacement within eight years, compared with about 6.5 per cent of the matched comparison group.

Longer use tracked with lower rates. Shorter use showed a smaller version of the same effect.

Two mechanisms are proposed, and both are believable. Less body weight means less load through the joint with every step — the knee carries three to four times body weight during ordinary walking, and more on stairs. And these medications appear to have anti-inflammatory effects beyond weight alone, which would matter in a disease that is partly inflammatory.

Now the caution. This was a retrospective database study, not a trial. The authors themselves note they could not account for physical activity, frailty, or how severe each person’s arthritis was at the start. People who are prescribed and stay on these medications for three years may differ from those who are not in ways the data cannot see. Association is not the same as cause, and a headline that says these drugs prevent knee replacements is running well ahead of what was measured.

The honest reading: for some people, sustained weight loss appears to delay or avoid an operation. That has always been true of weight loss. What is new is a medication that makes the loss achievable for people who could not previously get there.

Which is worth sitting with, if you have been told to lose weight before surgery and have found that advice impossible to act on while your knee hurts too much to exercise. That trap is real, and it is not a failure of willpower. Weight and knee load sets out what the target actually is — 5 to 7.5 per cent of body weight, not a goal weight — and why a modest change does more than people expect.

4. What these medications do not fix

Two things belong in the same conversation, because they bear directly on how a joint replacement goes.

Bone health. Research presented at the 2026 AAOS Annual Meeting, following 73,483 matched patients over five years, found higher rates of osteoporosis among GLP-1 users than matched comparators — 4.1 per cent against 3.2 per cent — along with smaller increases in gout. The investigators recommended bone health surveillance in at-risk patients. Bone quality matters in joint replacement, because implants are fixed into bone and have to stay fixed.

Muscle. A meaningful share of the weight lost on these medications is lean tissue rather than fat — published trials commonly report somewhere between a quarter and 40 per cent, and more at higher doses. That matters more for a knee replacement than for almost any other operation, because your quadriceps does most of the work of the recovery. The result of a knee replacement is decided less in theatre than in the eight weeks of rehabilitation that follow it, and rehabilitation is muscle work.

Neither is an argument against the medication. Both are arguments for what should accompany it: resistance training and adequate protein, deliberately, rather than weight loss on its own. Roughly 1.2 to 1.6 grams of protein per kilogram of body weight per day is the range commonly recommended alongside these medications, and it is worth getting specific advice on it from your GP or a dietitian.

If you are losing weight ahead of a knee replacement, you want to arrive at the operation lighter and stronger. Those are two separate projects.

What this means in practice

If you are on a GLP-1 medication and heading toward a hip or knee replacement:

• Tell your surgeon and anaesthetist. Name and dose, at the consultation, not on the day.
• Do not stop it unless you are specifically told to by the team looking after you.
• Expect the 24-hour clear fluid diet before your anaesthetic, and plan the day around it.
• Build in time if you can. Two to three months on the medication before surgery is where the evidence sits, rather than a fortnight.
• Add resistance training and protein, so the weight you lose is fat rather than the muscle you are about to need.
• Do not start or stop one on the strength of an article. That is a conversation with your GP or endocrinologist, weighed against your whole health rather than your knee.

The broader point is that these medications have changed who is a candidate for surgery and how well prepared they arrive. That is a good problem to have. It just needs to be planned for rather than discovered on the morning of the operation.

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If a replacement is on the horizon, knee replacement in Perth sets out how the practice approaches it, Do I Need a Knee Replacement? works through the decision, and non-surgical knee treatment covers what is worth trying first. Your surgery: before the operation sets out how preparation works, and knee replacement recovery covers what follows.

Dr Rhys Clark is a knee and hip replacement surgeon in Perth, consulting at Murdoch and Mandurah and operating at St John of God Murdoch, Sir Charles Gairdner Hospital and Perth Children’s Hospital.

To discuss your own situation, phone the rooms on (08) 6332 6365 or email info@rhysclark.com.au.