Can Weight Loss Injections Cause Blood Clots?

Step 1: Check what the regulators and trial data say

A question our prescribers hear most weeks is some version of this: "I read online that weight loss jabs cause blood clots, is that true?" The short answer is no, based on current evidence. Neither the MHRA nor the European Medicines Agency has identified a causal link between GLP-1 or dual GIP/GLP-1 medicines and venous thromboembolism (VTE). The Summary of Product Characteristics for both Mounjaro and Wegovy, published on the Electronic Medicines Compendium, lists the recognised adverse effects in detail; blood clots do not appear among them.

The large SURMOUNT-1 trial, which enrolled 2,539 adults over 72 weeks, did not flag VTE as a safety signal for tirzepatide. Likewise, the STEP programme for semaglutide produced no clotting signal across its thousands of participants. Post-marketing surveillance (which the MHRA runs continuously for all Black Triangle medicines) has not changed this position. That does not mean these medicines are without risk; it means the specific risk you are asking about is not currently supported by the evidence.

Where the picture becomes a little more nuanced is in observational research. Some large database studies published in journals such as the BMJ have looked at GLP-1 users at a population level and found no elevated VTE rate compared with other antiobesity medicines. If anything, a body of early mechanistic research suggests GLP-1 receptor activation may have modest anti-inflammatory effects on blood vessels, though this has not translated into a licensed cardiovascular-clot prevention indication for the injectable weight-loss formulations.

Step 2: Understand why obesity itself matters here

To make sense of the question, it helps to separate the medicine's effect from the condition it treats. Obesity is an independent risk factor for deep vein thrombosis (DVT) and pulmonary embolism (PE). Excess adipose tissue promotes a pro-inflammatory, pro-coagulant state; immobility associated with higher body weights adds to that risk. The NHS overview of obesity covers these cardiovascular and circulatory consequences clearly.

This matters for two reasons. First, some people starting a weight-loss injection will already carry a raised clotting risk simply because of their weight, that is not caused by the medicine, it pre-dates it. Second, if weight loss is sustained over months, the metabolic environment shifts: inflammation tends to fall, blood pressure often improves, and some of the background clotting risk reduces. The medicine's long-term effect on clotting risk is therefore more likely to be neutral or modestly beneficial than harmful, though this is not a licensed claim and individual circumstances vary.

Prescribers at a service like nume take a full medical history at consultation precisely because factors like prior DVT, inherited clotting disorders (such as Factor V Leiden), active cancer or prolonged immobility change the risk picture. None of these conditions are automatic disqualifiers, but they are conversations that must happen before a prescription is issued.

Step 3: Know which side effects are real and where to get help

GLP-1 and dual-agonist medicines do have a well-documented side-effect profile, just not one centred on blood clots. The effects most commonly reported are gastrointestinal: nausea, loose stools, constipation, reflux and reduced appetite, especially in the early weeks or after a dose step-up. These are listed clearly on the NHS tirzepatide medicines page and its semaglutide equivalent.

More serious but rare effects include acute pancreatitis. In January 2026 the MHRA issued a Drug Safety Update reinforcing that severe, persistent stomach pain (particularly pain that spreads to the back) warrants urgent medical attention and that treatment should be stopped while pancreatitis is ruled out. Gallbladder problems, significant dehydration from prolonged vomiting or diarrhoea, and hypersensitivity reactions are also on the recognised list. None of these overlap with the clotting-risk concern, but they are worth knowing.

If you are exploring the broader safety picture, the question of cancer risk with these medicines is another topic the evidence addresses, worth reading alongside this page for a fuller view. Suspected side effects of any kind can be reported directly to the MHRA via the Yellow Card scheme, which is open to patients as well as healthcare professionals.

Step 4: What this means if you take anticoagulants or have had a clot before

If you are already prescribed an anticoagulant (warfarin, apixaban, rivaroxaban or similar) starting a weight-loss injection raises a practical question rather than a contraindication. GLP-1 medicines slow gastric emptying, which can affect how quickly other oral medicines are absorbed. For warfarin in particular, any change in absorption timing can shift INR readings, so closer monitoring is usually recommended in the early titration period. There is a dedicated page on using weight-loss injections alongside blood thinners that covers this in more detail.

A history of DVT or PE is a relevant piece of medical history, not a reason to rule treatment out. Prescribers need to know about it, and about any thrombophilia or current anticoagulation, so they can assess the full picture. The weight-loss injection overview sets out how the consultation process works from the beginning. For many people with a history of clotting who carry excess weight, the net health benefit of treatment (including the downstream reduction in cardiovascular risk) is a factor that a prescriber will weigh alongside any theoretical concern.

If your question is more about blood donation while on these medicines, that is a separate topic covered on this page about giving blood on weight-loss injections. A related question that comes up just as regularly is whether B12 injections can cause weight loss, which is worth reading if you are comparing different injection-based options.