Short answerTwo large trials point in different directions. Aspirin worked as well as rivaroxaban when it followed a short course of rivaroxaban. Aspirin on its own did worse than enoxaparin injections in a larger Australian trial.
Not enough evidence yet
What it is
After a hip or knee replacement, blood clots can form in the leg veins and sometimes travel to the lungs. Most people are given a blood thinner for a period after surgery. Options include aspirin tablets, rivaroxaban tablets and enoxaparin injections. Which one is used differs between surgeons and hospitals.
What the research shows
enrolled 3,424 people having hip or knee replacement in Canada. Everyone took rivaroxaban for five days. They then continued rivaroxaban or switched to aspirin, for 9 days after knee replacement or 30 days after hip replacement. A symptomatic clot occurred in 0.64 percent of the aspirin group and 0.70 percent of the rivaroxaban group. Aspirin was not worse than rivaroxaban. Major bleeding was 0.47 percent with aspirin and 0.29 percent with rivaroxaban, which was not a significant difference.
was run in 31 Australian hospitals and enrolled 9,711 people having hip or knee replacement for osteoarthritis. Hospitals gave aspirin or enoxaparin injections from the start, for 35 days after hip replacement and 14 days after knee replacement. Within 90 days, a symptomatic clot occurred in 3.45 percent of the aspirin group and 1.82 percent of the enoxaparin group. Aspirin did not meet the test of being no worse than enoxaparin. Much of the difference came from clots below the knee.
The two trials asked different questions. In EPCAT II every person had rivaroxaban first and the aim was to compare the later part of treatment. In CRISTAL, aspirin or enoxaparin was used from the start. The trials also used different hospitals, people and comparison drugs.1,2
Risks and effort
All of these medicines can cause bleeding. Enoxaparin needs daily injections. Aspirin and rivaroxaban are tablets. The best choice depends on your risk of clots and bleeding, and your other medicines and conditions.
What usually works instead
Getting up and walking early after surgery is part of usual care, along with any compression device your hospital uses. Your surgeon will choose a blood thinner to suit you. See Total Knee Replacement and What is a Total Hip Replacement?.
Questions people ask
Is aspirin safe to use alone?
CRISTAL found more clots with aspirin alone than with enoxaparin. EPCAT II tested aspirin only after five days of rivaroxaban. Ask your surgeon what they use and why.
Should I take aspirin if I already take it for my heart?
Do not start or stop medicines on your own. Tell your surgical team about everything you take.
How long are blood thinners needed?
The trials used 9 to 35 days, depending on the operation. Your surgeon will advise.
Sources
Select a source to read what the study or guideline found, in plain words, with a link to the original.
Last reviewed 24 October 2026. Next review due October 2027.
