Do Varicose Veins Cause Blood Clots? What a 5-Fold Relative Risk and a 0.7% Absolute Risk Actually Mean

It is one of the most common worries brought into the consulting room:

"I've read that varicose veins cause blood clots. Isn't that dangerous?"

Many people arrive having seen the figure "five times" online. That figure is real. Read on its own, though, it sounds far more alarming than it is. Relative risk (how many times higher) and absolute risk (how likely it is to happen to you) are different numbers, and a judgement is only possible when you look at both.

This article puts the two side by side.

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The central evidence — 420,000 people followed in Taiwan

The largest study on this question was published in JAMA in 2018 by Chang and colleagues. Using Taiwan's National Health Insurance database, they followed 212,984 patients with varicose veins and 212,984 propensity-matched controls for a median of 7.5 years. Anyone who already had venous thromboembolism or peripheral artery disease at baseline was excluded.

Deep vein thrombosis (DVT)

	Incidence
Varicose veins	6.55 per 1,000 person-years	(10,360 people)
Controls	1.23 per 1,000 person-years	(1,980 people)

Hazard ratio 5.30 (95% CI 5.05–5.56)

Pulmonary embolism (PE)

	Incidence
Varicose veins	0.48 per 1,000 person-years	(793 people)
Controls	0.28 per 1,000 person-years	(451 people)

Hazard ratio 1.73 (95% CI 1.54–1.94)

How to read "5.3 times"

The two numbers must be read together.

Relative risk 5.30 — compared with people who do not have varicose veins.

Absolute risk 6.55 per 1,000 person-years — if 1,000 people with varicose veins live through one year, about 6.6 of them will develop a DVT. As a percentage, that is 0.66% per year.

Put the other way round: with varicose veins, the chance of not developing a DVT in a given year is 99.3%. The multiplier looks large because the control group's rate (0.12%) is very low to begin with. Five times a small number is still a small number.

For pulmonary embolism the difference is smaller still. The absolute rate was 0.48 per 1,000 person-years — 0.048% per year.

The caveat the authors themselves attached

There is a part of this paper that must not be left out when it is quoted. In their conclusion the authors wrote that the findings for pulmonary embolism and peripheral artery disease are less clear because of the potential for confounding, and that whether the association between varicose veins and DVT is causal, or reflects shared risk factors, requires further research.

In other words, what the study supports is not "varicose veins cause clots" but "clots are observed more often in people with varicose veins." Obesity, age and immobility could plausibly be driving both.

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The clot you are actually likely to meet is superficial

In practice, the thrombosis most often encountered with varicose veins is not in the deep veins but in the dilated veins just under the skin — superficial vein thrombosis (SVT). The vein becomes red, firm, cord-like and tender.

It is easy to dismiss as "just surface phlebitis." The French POST study, published in the Annals of Internal Medicine in 2010, changed that view. It enrolled 844 patients with symptomatic SVT at least 5 cm long.

At diagnosis, 210 of them (24.9%) already had a DVT or symptomatic pulmonary embolism as well.

The three-month follow-up of the 600 patients without DVT or PE at baseline matters too. Even though 90.5% of them received anticoagulants, 58 (10.2%) went on to have a thromboembolic complication:

• Pulmonary embolism 3 (0.5%)
• Deep vein thrombosis 15 (2.8%)
• Extension of the SVT 18 (3.3%)
• Recurrence of the SVT 10 (1.9%)

SVT, then, is not a condition that ends at the skin — it is a condition in which the deep system must be checked. That is why a hard, painful cord in the leg is scanned all the way down to the deep veins.

An unexpected finding — SVT without varicose veins was riskier

In the same study, the risk factors identified for complications at three months were male sex, a history of DVT or PE, a history of cancer — and the absence of varicose veins.

It sounds counter-intuitive, but the explanation is straightforward. An SVT in someone with varicose veins usually has an identifiable cause: a dilated vein. An SVT in a leg without varicose veins is more likely to have something else behind it that has not yet been found — an occult malignancy, a thrombophilia.

For patients with varicose veins this is reassuring. It also means that an SVT in a leg without varicose veins warrants a wider search for a cause.

How SVT is treated

A 2019 meta-analysis in Thrombosis and Haemostasis (17 studies, 6,862 patients) compared rates of thromboembolic complications by treatment.

• Fondaparinux: 1.4 per 100 person-years (95% CI 0.5–2.8) — the lowest
• Other treatments: 9.3–16.6 per 100 person-years
• No treatment or placebo: 10.5 per 100 person-years (95% CI 3.0–22.0)

Major bleeding was low and similar across groups. The authors were explicit that the quality of evidence for treatments other than fondaparinux is low, and the optimal treatment cannot be stated with certainty.

Anticoagulation is now the standard approach for saphenous SVT of 5 cm or more, as a 2018 review of the randomised trials also confirms.

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Situations in which the risk genuinely rises

Even with varicose veins, clot risk varies a great deal with circumstances. Two situations have solid evidence.

During cancer treatment In the Vienna Cancer and Thrombosis Study (1,270 patients, median follow-up 590 days), published in the Journal of Thrombosis and Haemostasis in 2013, cancer patients with varicose veins had a hazard ratio for venous thromboembolism of 2.10 (95% CI 1.29–3.41). The association held in multivariable analysis.

Before joint replacement surgery A 2023 meta-analysis in Phlebology (11 observational studies, 265,194 patients) found that patients with varicose veins undergoing arthroplasty had an odds ratio for venous thromboembolism of 2.37 (95% CI 1.54–3.63).

Both point the same way: varicose veins matter less as a risk on their own than when they coincide with another major risk situation. If you are facing surgery or chemotherapy, tell the treating doctor that you have varicose veins.

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Does treating the veins reduce clot risk?

Honestly: we could not find a study that answers this question convincingly.

The claim that "treating varicose veins reduces later DVT" circulates online. Within what we have been able to check, no large study supports it. It is too early to use it as a reason to recommend a procedure.

What is well measured is the opposite quantity — the clot risk that accompanies the procedure itself.

A 2019 Japanese nationwide survey in the Journal of Vascular Surgery: Venous and Lymphatic Disorders compiled the complications of 43,203 endovenous laser ablations.

Complication	Rate
Endovenous heat-induced thrombosis (EHIT) class 2	1.0%
EHIT class 3	0.11%
EHIT class 4	0.013%
Other deep vein thrombosis	0.063%
Pulmonary embolism	0.0067%

A 2020 Polish study followed 141 patients who underwent stripping with miniphlebectomy under prophylactic anticoagulation: postoperative DVT occurred in 3.5% (5 patients), all confined to the distal (calf) veins, with no proximal DVT and no pulmonary embolism.

In summary, the clot risk of a procedure is low but not zero. That is why risk is assessed before treatment and prophylactic anticoagulation is used where it is warranted.

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In summary

• With varicose veins, DVT risk is about 5.3 times higher. The absolute rate, however, is 0.66% per year — in any given year, 99.3% do not develop one.
• For pulmonary embolism the difference is smaller (hazard ratio 1.73, 0.048% per year).
• Causation has not been established. The authors themselves left open the possibility of shared risk factors.
• The clot you are more likely to encounter is superficial vein thrombosis (SVT), and one in four had a deep clot as well at the time of diagnosis. A hard, painful cord in the leg should be scanned.
• Extra care is needed when other risks coincide — cancer treatment, joint replacement surgery.
• There is no evidence yet that treatment prevents clots. Treatment is decided on symptoms and progression, not as clot prevention.

Varicose veins are neither "just unsightly veins" nor "a dangerous condition that leads to clots." Knowing the numbers accurately usually reduces the worry.

If you can feel a red, firm cord in the leg that hurts when pressed, do not wait — that is a situation that needs an ultrasound regardless of the statistics above.

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References

• Chang SL, Huang YL, Lee MC, et al. Association of Varicose Veins With Incident Venous Thromboembolism and Peripheral Artery Disease. JAMA. 2018;319(8):807-817. (PMID: 29486040)
• Decousus H, Quéré I, Presles E, et al. Superficial venous thrombosis and venous thromboembolism: a large, prospective epidemiologic study. Ann Intern Med. 2010;152(4):218-24. (PMID: 20157136)
• Duffett L, Kearon C, Rodger M, Carrier M. Treatment of Superficial Vein Thrombosis: A Systematic Review and Meta-Analysis. Thromb Haemost. 2019;119(3):479-489. (PMID: 30716777)
• Scovell SD, Ergul EA, Conrad MF. Medical management of acute superficial vein thrombosis of the saphenous vein. J Vasc Surg Venous Lymphat Disord. 2018;6(1):109-117. (PMID: 29097174)
• Königsbrügge O, Lötsch F, Reitter EM, et al. Presence of varicose veins in cancer patients increases the risk for occurrence of venous thromboembolism. J Thromb Haemost. 2013;11(11):1993-2000. (PMID: 24112869)
• Westby D, Ghoneim BM, Nolan F, et al. Varicose veins as a risk factor for venous thromboembolism in arthroplasty patients: Meta-analysis. Phlebology. 2023;38(3):150-156. (PMID: 36609190)
• Nemoto H, Mo M, Ito T, et al. Venous thromboembolism complications after endovenous laser ablation for varicose veins and role of duplex ultrasound scan. J Vasc Surg Venous Lymphat Disord. 2019;7(6):817-823. (PMID: 31540837)
• Wołkowski K, Wołkowski M, Urbanek T. Venous Thromboembolism Prophylaxis and Thrombotic Risk Stratification in the Varicose Veins Surgery-Prospective Observational Study. J Clin Med. 2020;9(12):3970. (PMID: 33297575)
• De Maeseneer MG, Kakkos SK, Aherne T, et al. Editor's Choice — European Society for Vascular Surgery (ESVS) 2022 Clinical Practice Guidelines on the Management of Chronic Venous Disease of the Lower Limbs. Eur J Vasc Endovasc Surg. 2022;63(2):184-267. (PMID: 35027279)