Can Anastrozole Cause Blood Clots? Why It Happens

Yes, anastrozole can increase the risk of blood clots, though the risk is relatively small. This happens because anastrozole lowers estrogen levels in the body, and estrogen helps keep blood vessels flexible and blood flowing smoothly. When estrogen drops, blood may become more likely to clot, especially in the legs (deep vein thrombosis) or lungs (pulmonary embolism). The connection is real, but it is not as strong as the blood clot risk from tamoxifen, another common breast cancer drug.

How Does Anastrozole Raise Blood Clot Risk?

Anastrozole works by stopping an enzyme called aromatase from turning androgens into estrogen. This is a targeted way to lower estrogen in postmenopausal women with hormone-sensitive breast cancer. Lower estrogen levels are the goal for treatment, but they also affect the lining of blood vessels.

Estrogen helps blood vessels stay dilated and prevents platelets from sticking together too easily. When estrogen drops, the inner lining of blood vessels, called the endothelium, may not work as well. This can make blood flow slower and increase the chance of a clot forming. Research published in the Journal of Clinical Oncology found that women taking anastrozole had a slightly higher rate of venous thromboembolic events compared to women not on any hormone therapy.

The risk is not huge. In large clinical trials, about 2 to 3 percent of women on anastrozole experienced a blood clot. That is compared to about 1 percent in women taking a placebo. So the absolute increase is small, but it is real.

Can Anastrozole Cause Blood Clots More Than Tamoxifen?

This is a common point of confusion. Tamoxifen, another breast cancer drug, has a higher blood clot risk than anastrozole. Studies have found that tamoxifen roughly doubles or triples the risk of blood clots in some women. Anastrozole does not raise the risk as much.

The ATAC trial, a large study comparing anastrozole to tamoxifen, showed that women on anastrozole had significantly fewer blood clots. In that study, about 2 percent of women on anastrozole had a clot, compared to about 4 to 5 percent on tamoxifen. That is why doctors sometimes switch women from tamoxifen to anastrozole if they have a history of clots or other risk factors.

But here is the honest truth: anastrozole is not risk-free. The risk is lower than tamoxifen, but it is still higher than no drug at all. If you have had a blood clot before, or if you have a condition like factor V Leiden that makes clotting more likely, your doctor will consider this carefully before prescribing anastrozole.

What Are the Symptoms of a Blood Clot on Anastrozole?

Knowing what to watch for matters because early treatment of a blood clot can prevent serious complications. The most common places for a clot to form are the deep veins of the leg, called deep vein thrombosis (DVT), or the lungs, called pulmonary embolism (PE).

For a DVT in the leg, look for these signs:

• Swelling in one leg, usually the calf or thigh
• Pain or tenderness that feels like a cramp or charley horse
• Warm skin on the affected leg
• Red or discolored skin

For a pulmonary embolism, symptoms can be more urgent:

• Sudden shortness of breath
• Chest pain that gets worse when you breathe deeply or cough
• Coughing up blood
• Feeling lightheaded or passing out

If you have any of these symptoms while taking anastrozole, do not wait. Go to an emergency room. Blood clots are treatable, but they can be fatal if ignored. The American Society of Clinical Oncology advises anyone on anastrozole to be aware of these symptoms and seek medical help immediately if they occur.

Who Is Most at Risk for Blood Clots on Anastrozole?

Not everyone on anastrozole has the same risk. Some factors make blood clots more likely. Your doctor will look at these before starting the drug.

Age is a factor. Women over 65 have a higher baseline risk for clots, and anastrozole adds to that. Obesity also increases risk because extra weight puts pressure on veins and can make blood flow slower. A history of previous blood clots is a major red flag. If you have had a DVT or PE before, anastrozole may not be the best choice.

Other risk factors include:

• Prolonged bed rest or immobility, like after surgery or a long flight
• Smoking, which damages blood vessels
• Certain genetic clotting disorders, like factor V Leiden
• Cancer itself, because some cancers increase clotting risk

If you have several of these risk factors, your doctor might consider a different aromatase inhibitor, like letrozole or exemestane, or they might recommend a blood thinner while you take anastrozole. Some studies suggest that the risk is similar across all aromatase inhibitors, but individual factors matter more than the specific drug.

Can You Prevent Blood Clots While Taking Anastrozole?

There is no guaranteed way to prevent blood clots on anastrozole, but you can lower your risk with some straightforward steps. Staying active is one of the most effective things you can do. Walking regularly keeps blood moving in your legs and reduces the chance of a clot forming.

If you have surgery planned, talk to your doctor about your anastrozole use. Some guidelines suggest stopping the drug for a short time before major surgery, though this decision should be made with your oncologist. Do not stop taking anastrozole on your own.

Staying hydrated is also important. Dehydration makes blood thicker and more likely to clot. Drink enough water throughout the day, especially if you are traveling or spending time in a warm environment. Avoid sitting for long periods. If you have a desk job or take a long flight, get up and walk every hour or so. Compression stockings can help some people, though evidence for their use during anastrozole therapy is limited.

Some people report that aspirin helps prevent clots, but do not start aspirin without asking your doctor. Aspirin can increase bleeding risk, and it is not proven to prevent the type of clots linked to anastrozole. Your doctor may prescribe a blood thinner like apixaban or rivaroxaban if your risk is high enough. This is a decision based on your personal health history, not a general recommendation.

Common Misconceptions About Anastrozole and Blood Clots

A few myths float around online about anastrozole and clotting. One is that anastrozole causes blood clots in everyone. That is not true. Most women on anastrozole will not get a clot. The risk is elevated, but it is still a minority of patients.

Another myth is that if you had no clots on tamoxifen, you will have no clots on anastrozole. That is not how it works. The two drugs affect the body differently. Some women have clots on one drug but not the other. Your history on tamoxifen does not predict your risk on anastrozole perfectly.

There is also a claim that taking vitamin E or fish oil can prevent clots from anastrozole. As of 2026, there is no clinical evidence that these supplements reduce the specific clotting risk from anastrozole. Some supplements can even interfere with how the drug works. Always tell your doctor about any supplements you take.

Finally, some people think that if they feel fine, they do not need to worry about clots. But many blood clots start with no symptoms. That is why being aware of the warning signs is important even if you feel healthy. The risk is present from the first dose, though it may be highest in the first few months of treatment.