Last reviewed: September 29, 2026

For most people having a routine knee replacement, it appears to be, though the studies do not all agree. For people at higher risk of clots, the evidence is thin.

After a knee replacement, blood clots can form in the veins of the leg and sometimes travel to the lungs. Nearly everyone is given something to lower that risk. Aspirin is one option. Prescription blood thinners, taken as pills or injections, are the others.

Several pooled reviews of trials found that aspirin prevented clots about as well as prescription blood thinners, with no clear difference in bleeding, wound problems or death. A large Canadian trial published in 2026 found that aspirin alone from the start worked as well as a short course of a blood-thinning pill followed by aspirin, with about the same bleeding.

A large Australian trial points the other way. Hospitals using aspirin had more clots than hospitals using a blood-thinning injection, mostly smaller clots below the knee. Deaths and major bleeding were no different.

The large trials enrolled mostly people at ordinary risk. Two large US record studies of people who had had a clot before found no more clots with aspirin than with prescription blood thinners. But surgeons chose who received aspirin, so those patients may have been at lower risk to begin with. Some reviews advise against aspirin alone for people at higher risk. Studies disagree on whether severe obesity on its own raises the risk of serious clots. No study found has tested aspirin after a knee replacement specifically in people with active cancer or an inherited clotting disorder.

Guidelines differ. The American Academy of Orthopaedic Surgeons guideline, from 2011, recommends a medicine or a compression device but found the evidence too unclear to name the best medicine. The American Society of Hematology says either aspirin or a blood thinner is reasonable, but rates the evidence as very low certainty. Neither makes a separate recommendation for people at higher risk. British guidance lists aspirin as one option.

In general:

  • Which medicine to use is decided with your surgeon, based on your own risk of clots and of bleeding.
  • Tell your surgeon about any past blood clot, cancer, clotting disorder or bleeding problem.
  • Pain, swelling or tenderness in the calf needs to be seen promptly.
  • Sudden shortness of breath or chest pain needs emergency care.

Drafted with AI assistance from published sources, reviewed and edited by Babak Sheikh, MD.

Sources:

  • Matharu GS, et al. Clinical Effectiveness and Safety of Aspirin for Venous Thromboembolism Prophylaxis After Total Hip and Knee Replacement: A Systematic Review and Meta-analysis of Randomized Clinical Trials. JAMA Intern Med. 2020. https://doi.org/10.1001/jamainternmed.2019.6108
  • Hong Z, et al. Aspirin Is as Effective and Safe as Oral Anticoagulants for Venous Thromboembolism Prophylaxis After Joint Arthroplasty: A Systematic Review and Meta-Analysis of Randomized Clinical Trials. J Bone Joint Surg Am. 2025. https://doi.org/10.2106/JBJS.24.00946
  • Ren Y, et al. Aspirin Is as Effective as Oral Anticoagulants for Venous Thromboembolism Prophylaxis After Knee Arthroplasty: A Systematic Review and Meta-Analysis of Randomized Controlled Trials. J Arthroplasty. 2025. https://doi.org/10.1016/j.arth.2025.02.052
  • Shivakumar S, et al. Rivaroxaban Then Aspirin vs. Aspirin Alone After Total Hip or Knee Arthroplasty. N Engl J Med. 2026. https://doi.org/10.1056/NEJMoa2603649
  • Anderson DR, et al. Aspirin or Rivaroxaban for VTE Prophylaxis After Hip or Knee Arthroplasty. N Engl J Med. 2018. https://doi.org/10.1056/NEJMoa1712746
  • CRISTAL Study Group, Sidhu VS, et al. Effect of Aspirin vs Enoxaparin on Symptomatic Venous Thromboembolism in Patients Undergoing Hip or Knee Arthroplasty: The CRISTAL Randomized Trial. JAMA. 2022. https://doi.org/10.1001/jama.2022.13416
  • CRISTAL Study Group. Effect of Aspirin vs Enoxaparin on 90-Day Mortality in Patients Undergoing Hip or Knee Arthroplasty: A Secondary Analysis of the CRISTAL Cluster Randomized Trial. JAMA Netw Open. 2023. https://doi.org/10.1001/jamanetworkopen.2023.17838
  • Liu KC, et al. Safety and Efficacy of Aspirin Compared to Enoxaparin Following Total Hip and Total Knee Arthroplasty Among Patients Who Have a History of Venous Thromboembolic Disease. J Arthroplasty. 2025. https://doi.org/10.1016/j.arth.2024.11.029
  • Verhey JT, et al. History of Venous Thromboembolism Is Not a Contraindication to Aspirin Prophylaxis in Patients Undergoing Total Joint Arthroplasty. J Arthroplasty. 2026. https://doi.org/10.1016/j.arth.2025.10.069
  • Riediger C, et al. Aspirin for Venous Thromboembolism Prevention in Orthopaedic Surgery With Focus on Trauma and Arthroplasty: A Structured Evidence-Based Review of Randomised Trials, Guidelines, and Contemporary Practice Considerations. J Clin Med. 2026. https://doi.org/10.3390/jcm15124550
  • Sloan M, et al. Is Obesity Associated With Increased Risk of Deep Vein Thrombosis or Pulmonary Embolism After Hip and Knee Arthroplasty? A Large Database Study. Clin Orthop Relat Res. 2019. https://doi.org/10.1097/CORR.0000000000000615
  • Thompson R, et al. Are Patients With Morbid Obesity at Increased Risk of Pulmonary Embolism or Proximal Deep Vein Thrombosis After Lower Limb Arthroplasty? A Large-Database Study. Clin Orthop Relat Res. 2024. https://doi.org/10.1097/CORR.0000000000002742
  • Jacobs JJ, et al. American Academy of Orthopaedic Surgeons Clinical Practice Guideline on: Preventing Venous Thromboembolic Disease in Patients Undergoing Elective Hip and Knee Arthroplasty. J Bone Joint Surg Am. 2012. https://doi.org/10.2106/JBJS.9408.ebo746
  • Anderson DR, et al. American Society of Hematology 2019 Guidelines for Management of Venous Thromboembolism: Prevention of Venous Thromboembolism in Surgical Hospitalized Patients. Blood Adv. 2019. https://doi.org/10.1182/bloodadvances.2019000975
  • National Institute for Health and Care Excellence. Venous Thromboembolism in Over 16s: Reducing the Risk of Hospital-Acquired Deep Vein Thrombosis or Pulmonary Embolism (NG89). https://www.nice.org.uk/guidance/ng89

This is general information about what published research shows. It is not medical advice and it is not about your situation. Reading it does not create a doctor-patient relationship with me. I am a retired orthopedic surgeon and I do not see patients. Talk with your own doctor before changing anything about your care. If something feels suddenly or seriously wrong, get medical care promptly rather than waiting.

This page is provided as is. I make no guarantee that it is accurate, complete or current, and medical evidence changes. Use of it is at your own risk.

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