Blood Clots: What Raises Your Risk, What Lowers It, and What Not to Mix With Your Blood Thinner

A clot in a leg vein (a DVT, deep vein thrombosis) or one that travels to the lungs (a pulmonary embolism, PE) is one of the commonest serious things that can happen after surgery, a long journey or a spell of illness. Here’s what actually raises and lowers the risk, graded, what helps after a clot, and why the supplement aisle matters more than most people think if you’re on a blood thinner.

The grades in this post: Strong means good randomised trials agree. Mixed means trials disagree or are small and short. Mechanistic means the biology makes sense but it hasn’t been shown to change outcomes in people. None means there’s no usable evidence in people. Large observational studies are labelled as such: they show a link, not proof of cause.

What raises the risk

Clots form when three things line up: blood moving slowly, blood more ready to clot than usual, and damage to a vessel wall. Most risk factors push one of those.

What about alcohol? Three large US cohorts (217,442 people, followed for 20 years or more) found no link between how much or how often people drank and their risk of PE (Harrington 2018). None found: observational and null.

The HRT point is worth knowing. If you’re choosing HRT and clots are a worry, the route matters as much as the hormone. That’s a conversation with your GP or menopause clinic, and the evidence gives you a good question to ask. More in HRT: the complete clinical picture.

What lowers it

Moving. Across 14 cohort studies and 1.3 million people, the most active had about 13% lower risk of a clot than the least active, and the link didn’t depend on body weight (Kunutsor 2020). Observational, consistent. Not dramatic, but it’s another reason to keep moving, and it costs nothing.

On long flights: compression stockings. In 12 trials of flights lasting more than 5 hours, stockings cut clots found on a leg scan to about a tenth: 3 people with stockings had one, against 47 without, among 2,637 passengers (Clarke 2021, Cochrane). In the review’s plain-language summary, that’s a fall in symptomless clots “from a few tens per thousand passengers to two or three per thousand”. They also reduced ankle swelling. Three things temper that:

• These were symptomless clots, found only because everyone was scanned. Nobody in any trial had a clot they could feel, a PE or a death, so the trials can’t show whether stockings prevent the events that matter.
• Much of the evidence comes from one research group (the LONFLIT trials, several using one brand of flight sock). An independent UK trial found the same direction: 12 of 116 over-50s without stockings had a symptomless calf clot, and none of those wearing them did (Scurr 2001).
• Clot rates varied a lot between trials: in three of them, nobody had one at all.

Strong for symptomless clots (Cochrane rates it high certainty; I’d call it moderate to high). None for symptomatic clots or PE. Stockings are cheap and low-risk, so for anyone with other risk factors on a long-haul flight they’re a sensible step. Get them properly fitted.

Move during the journey, too: get up, walk the aisle, work the calves when seated. That’s sensible advice from travel guidance; I haven’t found a trial that tests it on its own. Practice, not trial evidence.

Drink water? The NHS lists being dehydrated as one of the times clot risk rises, and drinking enough is harmless. I haven’t found a trial showing that drinking extra prevents clots. None found.

After a clot: moving again is safe

People are often frightened to exercise after a DVT or PE, as if movement might dislodge something. The evidence points the other way.

• Walking can start straight away, once anticoagulation has begun. Across 13 studies and 3,269 people with a new DVT, walking early (rather than bed rest) did not increase new PEs, clot growth or deaths, and people with moderate or severe leg pain got relief sooner (Liu 2015). Mixed A meta-analysis of trials and cohort studies. Bed rest used to be standard; it isn’t now.
• Across 10 studies (7 trials, 3 cohorts), exercise after a DVT was safe, improved quality of life, reduced pain and lowered the severity of post-thrombotic syndrome (the long-term leg pain and swelling some people get) (Rook 2023). Mixed Small trials and cohorts.
• A 2026 review of 4 trials (250 people) starting structured exercise within 3 months of a clot found no safety problems; benefits over usual care weren’t yet clear in such small numbers (Steiner 2026). Mixed Small RCTs.

How soon, in practice: walking from the first days on treatment; structured exercise (a walking programme, cycling, strength work) built up over the following weeks, and started within the first 3 months in the trials, with your clinician’s agreement. A PE is more individual: breathlessness sets the pace, so check with your team first. Heavy straining and contact sport while on a blood thinner are different questions (the risk is bleeding from a knock, not the clot), so ask your team.

If you’re on a blood thinner: the supplement aisle matters

This is where I most often see a gap. Someone is on warfarin or a newer anticoagulant (apixaban, rivaroxaban and others), and they’ve also added fish oil, turmeric, ginkgo or a “blood-thinning” enzyme because they read it was good for circulation.

Never stop, start or change an anticoagulant yourself. Talk to the prescriber or pharmacist first, and tell them about every supplement, including ones you think of as food.

• Warfarin and vitamin K: consistency, not avoidance. Warfarin works against vitamin K. Big swings in intake (a week of kale smoothies, then none) move your INR. The answer is to eat green vegetables steadily, not to cut them out. Some over-the-counter multivitamins contain enough vitamin K to shift it (Rohde 2007, review). Strong Consistent evidence. Natto is exceptionally high in vitamin K2: see my nattokinase article.
• Herbs and warfarin. A 2020 review found 78 herbs, foods and supplements reported to interact with warfarin, including ginkgo, St John’s wort, cranberry, green tea, ginger and chamomile tea, with bleeding events up to and including fatal brain bleeds (Tan 2020). The controlled trials are few, small and mostly low quality, and most didn’t show a clear effect on how warfarin works (Choi 2017, 9 trials). Mixed Mostly case reports; some small trials. The practical rule doesn’t change: tell your clinician.
• Fish oil (omega-3). Across 11 trials and 120,643 people, ordinary omega-3 supplements didn’t increase bleeding. High-dose purified EPA did raise it slightly (about 0.6% in absolute terms) (Javaid 2024). Those trials looked at bleeding in general, not at fish oil combined with an anticoagulant, so it still belongs on the list you tell your clinician about. Strong Meta-analysis of trials.
• Nattokinase. Sold as a “clot-buster”. A 3-year trial found no change in clotting or clot breakdown, and there’s a published case of a brain bleed when it was combined with aspirin. Full details in my nattokinase article. If it worked the way it’s marketed, it would carry a drug’s bleeding risk.

The newer anticoagulants (DOACs) don’t depend on vitamin K, so food matters less. But they still interact with some medicines and herbal remedies, and St John’s wort is the clearest example. The BNF lists it against apixaban, rivaroxaban, edoxaban and dabigatran: St John’s wort “is predicted to decrease the exposure” to each, so the drug may protect you less against a clot. It rates each interaction severe, based on study evidence; the manufacturers’ advice ranges from caution (edoxaban) to avoid (dabigatran). If you take one of these, don’t start St John’s wort, or any herbal remedy, without asking your pharmacist (BNF interactions, read 13 October 2026).

Where testing fits

There’s no supplement or functional test that tells you your clot risk. What does help:

• Knowing your own risk factors (above) and planning for the high-risk moments: surgery, long-haul travel, a new prescription.
• On warfarin: your INR checks are the test that matters.
• A personal or strong family history of clots is a question for your GP; inherited clotting conditions are tested through the NHS when they’re warranted.

How strong is the evidence?

Sources

1. Chandra D et al. Meta-analysis: travel and risk for venous thromboembolism. Ann Intern Med 2009;151:180–90. PMID 19581633. doi:10.7326/0003-4819-151-3-200908040-00129
2. Choi S et al. A systematic review of the pharmacokinetic and pharmacodynamic interactions of herbal medicine with warfarin. PLoS One 2017;12:e0182794. PMID 28797065. doi:10.1371/journal.pone.0182794
3. Clarke MJ et al. Compression stockings for preventing deep vein thrombosis in airline passengers. Cochrane Database Syst Rev 2021;4:CD004002. PMID 33878207. doi:10.1002/14651858.CD004002.pub4
4. de Bastos M et al. Combined oral contraceptives: venous thrombosis. Cochrane Database Syst Rev 2014;3:CD010813. PMID 24590565. doi:10.1002/14651858.CD010813.pub2
5. Harrington LB et al. Alcohol consumption and the risk of incident pulmonary embolism in US women and men. J Thromb Haemost 2018;16:1753–62. PMID 29974610. doi:10.1111/jth.14224
6. Javaid M et al. Bleeding risk in patients receiving omega-3 polyunsaturated fatty acids: a systematic review and meta-analysis of randomized clinical trials. J Am Heart Assoc 2024;13:e032390. PMID 38742535. doi:10.1161/JAHA.123.032390
7. Kunutsor SK et al. Physical activity and risk of venous thromboembolism: systematic review and meta-analysis of prospective cohort studies. Eur J Epidemiol 2020;35:431–42. PMID 31728878. doi:10.1007/s10654-019-00579-2
8. Liu Z et al. Bed rest versus early ambulation with standard anticoagulation in the management of deep vein thrombosis: a meta-analysis. PLoS One 2015;10:e0121388. PMID 25860350. doi:10.1371/journal.pone.0121388
9. BNF (British National Formulary), NICE. Interactions: apixaban, rivaroxaban, edoxaban and dabigatran, each with St John’s wort. Read 13 October 2026. bnf.nice.org.uk
10. NHS. Deep vein thrombosis (DVT). Page last reviewed 30 April 2026; read 13 October 2026. nhs.uk
11. NHS. Pulmonary embolism. Page last reviewed 25 May 2023; read 13 October 2026. nhs.uk
12. Rohde LE et al. Dietary vitamin K intake and anticoagulation in elderly patients. Curr Opin Clin Nutr Metab Care 2007;10:1–5. PMID 17143047. doi:10.1097/MCO.0b013e328011c46c
13. Rook B et al. Effect of exercise after a deep venous thrombosis: a systematic review. J Eur Acad Dermatol Venereol 2024;38:289–301 (online 2023). PMID 37731155. doi:10.1111/jdv.19523
14. Scurr JH et al. Frequency and prevention of symptomless deep-vein thrombosis in long-haul flights: a randomised trial. Lancet 2001;357:1485–89. PMID 11377600. doi:10.1016/S0140-6736(00)04645-6
15. Steiner D et al. Structured exercise training and rehabilitation in patients with acute venous thromboembolism: a systematic review of randomized controlled trials. Eur J Prev Cardiol 2026. PMID 41790033. doi:10.1093/eurjpc/zwag096
16. Tan CSS, Lee SWH. Warfarin and food, herbal or dietary supplement interactions: a systematic review. Br J Clin Pharmacol 2021;87:352–74 (online 2020). PMID 32478963. doi:10.1111/bcp.14404
17. Vinogradova Y et al. Use of hormone replacement therapy and risk of venous thromboembolism: nested case-control studies using the QResearch and CPRD databases. BMJ 2019;364:k4810. PMID 30626577. doi:10.1136/bmj.k4810