The question arrives with genuine confusion attached. These drugs slow the gut — everyone knows that, it causes constipation, it is why the anaesthetic guidance changed. So why is a meaningful share of people dealing with the opposite?
Because in most cases the diarrhoea is not being caused by slowed transit. It is being caused by something else that arrived at the same time.
The four causes worth knowing
Bile acid malabsorption. The most under-recognised and the most treatable. Bile acids are normally reabsorbed in the terminal ileum; when that process is disrupted — by rapid weight loss, altered gallbladder function, or gallbladder removal — they spill into the colon, where they are a potent secretory stimulus. The stool is typically watery, urgent, often yellow, and characteristically worse after a fatty meal or in the morning. It responds well to a bile acid sequestrant such as colestyramine, which is a prescription and a specific answer rather than a general one.
This is closely related to the gallstone story, which is itself a consequence of losing weight quickly.
Metformin. Enormously common in this population and a well-known cause of diarrhoea in its own right. If you started both around the same time, the metformin is at least as likely a culprit, and a switch to the extended-release formulation frequently resolves it. This is worth raising specifically, because people tend to blame the newer drug.
Sugar alcohols and sweeteners. Sorbitol, maltitol, xylitol and erythritol are osmotic laxatives at sufficient dose, and they are everywhere in the protein bars, shakes and low-carb products people on these drugs suddenly start eating in quantity. This cause is often solved by reading one label.
Overflow around constipation. Counterintuitive and genuinely common. Hard stool blocks the rectum and liquid passes around it, presenting as diarrhoea. Treating it as diarrhoea makes it worse. The clue is a pattern of constipation preceding it, or leakage without a proper bowel movement.
~9%
Diarrhoea reported at diabetes doses of Ozempic
FDA prescribing information
~30% vs 16%
Diarrhoea on semaglutide 2.4 mg versus placebo in STEP 1
NEJM, 2021
Yellow, urgent
Character suggesting bile acid malabsorption rather than ordinary drug effect
What to do first
Look at everything else you started. Metformin, a new protein product, a magnesium supplement taken for constipation, a fibre supplement introduced too fast, a sudden increase in artificial sweeteners. One of these explains a large share of cases.
Replace fluid and salt, not just fluid. Diarrhoea removes sodium and potassium as well as water, and plain water alone leaves people flat. An oral rehydration solution is the right tool. This matters more than it sounds — it is the direct route to the acute kidney injury that puts people on a drip.
Eat small, low-fat and low-fibre for a few days. Rice, toast, banana, potato, lean protein. Then return to normal food — prolonged restriction is its own problem, and you cannot afford to lose more protein than necessary.
Reduce fat temporarily. Fat is the trigger if bile acids are involved, and it slows emptying in a way that makes everything else worse.
Consider holding the dose rather than escalating. Extending a step to six or eight weeks resolves a great deal — the intervals are minimums, as set out in the GLP-1 dose charts.
Loperamide, occasionally. Reasonable for short-term control. Not a long-term answer, and it should not be used if there is fever or blood in the stool.
The alternating pattern
A significant number of people get both — constipated for days, then loose, then constipated again.
That usually means the underlying problem is disordered motility rather than either symptom individually, and treating each one as it appears produces a swinging pattern that never settles. The stable version is a consistent routine: adequate fluid, a steady soluble fibre intake rather than sporadic large doses, and an osmotic laxative used regularly rather than a stimulant used in emergencies. The full approach is in constipation on a GLP-1.
Before you blame the injection, look at the metformin, the protein bar and the sweetener. Between them they explain most of it.
When it needs assessment
Persistent diarrhoea also warrants a check for coeliac disease, inflammatory bowel disease and microscopic colitis — all of which exist independently of your medication and can happen to coincide with starting it. Attributing a new symptom to a drug is one of the more common ways a diagnosis gets delayed by a year.
The timeline
Like most gut symptoms in this class, worst in the first weeks and after each dose step, easing within four to eight weeks at a stable dose. The general pattern across every symptom is in how long side effects last.
What is different about diarrhoea is that a larger proportion of cases turn out to have a specific, findable cause than is true of nausea. It is worth looking rather than waiting.
The summary
Uncommon at diabetes doses, considerably more common at weight-management doses, and usually caused by something other than the injection itself.
Audit what else changed, replace salt as well as water, hold the dose rather than climbing, and ask about bile acids if it is yellow and urgent. Most of it resolves. The bit that does not is usually a diagnosis waiting to be made.