Zepbound® and blood clots

What the Zepbound® documents say

A search of the US label, the Canadian monograph and the EU product information for thrombosis, embolism and clot found no adverse reaction entry ^[1][3][4]. The only related line concerns warfarin: because Zepbound® delays stomach emptying, absorption of oral medicines can change, and the label asks prescribers to monitor medicines with a narrow therapeutic index, warfarin being its example ^[1]. The EU information says the same and adds that the effect is most pronounced when tirzepatide is started and after dose increases ^[4].

Two other listed findings are worth knowing because they overlap with clot symptoms. Zepbound® raised heart rate by 1 to 3 beats per minute on average and caused low blood pressure in 1.6% against 0.1% on placebo ^[1]. A lung clot can also cause a fast heart rate, dizziness and low blood pressure ^[15], so these should not be written off as side effects when they arrive suddenly with chest pain or breathlessness.

What large studies show

Clots are too rare for the SURMOUNT trials to answer the question, so the main evidence comes from health records. In a study using the TriNetX global records network, 701,374 adults with obesity starting tirzepatide or a GLP-1 medicine were matched on their characteristics. After one year, tirzepatide was linked to a lower risk of venous thromboembolism (hazard ratio 0.90), driven by fewer pulmonary emboli (0.88); deep vein thrombosis did not differ ^[5]. The authors stress that the design cannot establish cause ^[5].

A second records study compared people with type 2 diabetes and overweight or obesity starting tirzepatide with matched people on lifestyle treatment alone, and found markedly lower 12-month rates of pulmonary embolism and deep vein thrombosis on tirzepatide, with no difference for superficial vein clots ^[6]. Comparisons like this can be skewed by who gets prescribed a new medicine. An analysis of FDA spontaneous reports from 2020 to 2025 found that GLP-1 medicines overall were not disproportionately reported for clotting events and that tirzepatide had the lowest reporting ratio among them; spontaneous reports cannot give rates ^[7].

What SURMOUNT-1 measured in the blood

A 2026 analysis of stored samples from SURMOUNT-1, the trial behind Zepbound®’s weight indication, measured markers of inflammation, blood vessel lining and clotting in about 100 people per group over 72 weeks ^[8]. Compared with placebo, tirzepatide lowered plasminogen activator inhibitor-1, a protein that slows the breakdown of clots, by 36% to 44% across doses, and lowered C-reactive protein, a marker of inflammation, by 37% to 55% ^[8]. Platelets fell slightly only at 15 mg, and fibrinogen, tissue plasminogen activator and thrombomodulin showed no consistent change ^[8].

These are laboratory markers in a subset of trial participants, analysed after the fact and funded by the manufacturer. They point in a reassuring direction but do not show that Zepbound® prevents clots ^[8].

The case reports, and what they can and cannot show

Three published cases are often cited. A 20-year-old man with a body mass index above 35, taking tirzepatide for weight loss, lost 12 kg in six weeks and developed an extensive clot in the left leg; tests found no clotting disorder, and the authors suggested the rapid weight loss as a possible factor ^[9]. A woman in her early forties developed a lung clot about 20 days after starting tirzepatide, with no other identified risk factors ^[10]. Another woman developed an unprovoked leg clot within months of starting, which improved with blood thinners and stopping the medicine ^[11].

Single cases show that a clot happened while someone was taking the medicine. They cannot show that the medicine caused it, or how often this occurs among the large number of people using tirzepatide, many of whom already carry clot risk factors.

Why the background risk matters

Three layers, from best to least supported.

• Established risk factors. NHLBI lists obesity, diabetes, high blood pressure, heart and kidney disease, cancer, infections, older age and long periods without moving among causes of venous clots, and notes the risk nearly doubles every decade after 40 ^[13]. CDC adds major surgery, hospital stays, estrogen-containing contraceptives, hormone therapy and pregnancy ^[14]. Many people prescribed Zepbound® have several of these.
• Plausible links to treatment. Vomiting and diarrhea on Zepbound® can cause dehydration ^[1], and days spent unwell often mean less movement. These are reasonable concerns, but no study has shown that they raise clot risk on Zepbound®.
• Proposed but unproven. Authors of one case report suggested that rapid weight loss itself might contribute ^[9]; this has not been tested in a trial.

Situations that need extra attention

Surgery combines two Zepbound®-related issues. Major surgery is a clot risk factor ^[14], and Zepbound® carries a separate warning about aspiration under general anaesthesia or deep sedation, so the label asks you to tell your providers about any planned procedure ^[1]. Long journeys are another: NHLBI notes that clots can develop during flights longer than four hours, especially with other risk factors ^[13].

If you take warfarin, your prescriber may want closer monitoring when you start Zepbound® and after each dose step ^[1][4]. If you use estrogen-containing birth control, CDC counts that as a clot risk factor ^[14]; how Zepbound® affects the pill’s absorption is covered on the oral contraceptives page.

What may help

MedlinePlus suggests moving your legs often during long plane or car trips and other times you sit or lie still for long, not smoking, and taking any blood thinner your provider prescribes ^[12][15]. The Zepbound® Medication Guide asks you to drink fluids to help prevent dehydration and to report nausea, vomiting or diarrhea that persists without delay ^[2].

If you have had a clot before, or have a family history of clots, tell your prescriber before starting Zepbound®, since both raise risk regardless of medicine ^[14].

Zepbound® and Mounjaro® are registered trademarks of Eli Lilly and Company. PepLabo is not affiliated with or endorsed by Eli Lilly and Company.