Wegovy and Blood in Your Stool: What's Going On and What to Do

You noticed something in the toilet — here is the likely picture

You checked after going to the loo, saw something that looked like blood, and now you are wondering whether Wegovy is responsible. That moment of noticing is actually a useful checking habit: a quick look before you flush takes seconds and can pick up changes that matter clinically. So it is worth doing regularly while you are on any GLP-1 treatment.

The most common explanation for bright-red blood during Wegovy treatment is mechanical rather than pharmacological. Semaglutide significantly slows how quickly food moves through your gut. For many people, that results in changes in stool consistency that swing between loose and firm, and a proportion of patients develop constipation. Straining against a hard stool can cause small tears or fissures around the anal canal, or aggravate haemorrhoids that were already there. Both produce the classic bright-red blood you might see on the paper or at the surface of the stool. Painful to pass, fresh red at the margins, that pattern is almost always a lower-bowel source.

That does not mean you should dismiss it. Even a mechanical cause needs attention if it persists, because repeated straining or unresolved haemorrhoids are treatable and should not simply be tolerated. The NHS medicines information for semaglutide lists constipation and diarrhoea among the more common reactions, and either can create conditions that produce minor bleeding. If you have noticed what looks like bloody stool while taking Wegovy, understanding whether the cause is mechanical or something requiring urgent review is an important first step. What matters is the pattern, the colour, and whether it is accompanied by other symptoms.

The colour of the blood tells you where to focus your concern

Colour is your most immediate signal. Bright-red blood, appearing on the surface of the stool or on tissue, points to the lower part of the digestive tract: the rectum, anal canal or sigmoid colon. It is more often a local, mechanical issue (treatable, rarely urgent in isolation) but it still needs your prescriber or GP to know about it.

Dark blood mixed through the stool, or stool that is black and tar-like, is a different matter entirely. That appearance suggests the bleeding is happening higher up (in the stomach or small intestine) where blood has time to oxidise before it exits. This is the kind of finding described on the Wegovy and dark stool page in more detail, and it warrants same-day assessment, not a wait-and-see approach. Call 111, contact your GP urgently, or go to A&E if there is any doubt.

There is also the question of pancreatitis. In January 2026 the MHRA issued a Drug Safety Update reminding prescribers and patients that acute pancreatitis is a known, infrequent but serious risk with GLP-1 medicines. If you are concerned about whether Wegovy can cause black stool, that page explains the specific circumstances in which dark or tarry stools may appear and what to do if they do. That combination is not a side effect to manage at home.

You can read more about how semaglutide affects the gut overall on the Wegovy treatment page, including the broader side-effect picture.

When to contact someone, and who that should be

The threshold for getting advice is lower than most people think. If you see blood in your stool more than once, or if it is accompanied by abdominal pain, a significant change in how often you are going, unexplained fatigue, or any stool that is dark or foul-smelling beyond the usual, speak to a clinician the same day. The NHS patient-level guidance on semaglutide side effects sets out which symptoms should trigger an urgent call to your doctor or pharmacist; that page is worth bookmarking.

If you are an existing patient, our prescribers are available for aftercare seven days a week through the contact page. Reach out directly rather than adjusting your dose yourself. Stopping semaglutide abruptly or reducing the dose without guidance is not the right response to a GI symptom, and in most cases it will not be necessary once the cause is identified.

There are also wider questions worth considering. Rectal bleeding that is new in a person over 40 is one of the criteria the NHS uses to trigger a bowel cancer referral. Wegovy does not cause colorectal cancer, but it should not be used as an explanation that delays an investigation that would otherwise be clinically appropriate. If your GP would normally refer you based on your age and the symptom, that referral should still happen.

For people thinking about whether Wegovy is right for them in the first place, the weight-loss treatments page covers what a full clinical assessment involves, including how prescribers at nume review your health history, not just your BMI, before any treatment is approved. The full side-effect and safety picture is also covered on the semaglutide and blood in stool page if you want more context on the mechanism.

What this means if you are deciding whether to continue treatment

A single episode of minor bright-red bleeding, in the context of constipation that started with semaglutide, is unlikely on its own to be a reason to stop treatment. But that call belongs to a prescriber, not a web page. The right response is to report it, let it be assessed, and follow the clinical advice that comes from that conversation.

If a gut-side-effect pattern is making Wegovy hard to tolerate, there is a wider set of questions about dose timing, dietary adjustments, and whether treatment should continue at the current strength. These are exactly the conversations a prescriber is there for. You can also explore how Wegovy compares to other licensed options on the weight-loss overview page.

Speak to our prescribers if you have noticed blood in your stool during treatment, they can review your situation and advise on next steps without you needing to wait for a GP appointment.