Letrozole can raise blood pressure. In clinical trials, about 8% of patients taking letrozole developed hypertension, and studies comparing women on aromatase inhibitors like letrozole to healthy postmenopausal women found a meaningful difference: an average systolic blood pressure of 128.6 mmHg versus 116.2 mmHg in controls. The effect isn’t universal, but it’s real enough that the American Heart Association includes letrozole on its list of cancer therapies that can increase blood pressure.
Why Letrozole Affects Blood Pressure
Letrozole works by blocking an enzyme called aromatase, which your body uses to produce estrogen. For breast cancer treatment, that’s the point: starving hormone-sensitive tumors of the estrogen they need to grow. But estrogen also plays a protective role in your cardiovascular system. It helps keep blood vessels flexible and regulates a hormonal system in the kidneys (the renin-angiotensin system) that controls blood pressure.
When letrozole suppresses estrogen, those protective effects diminish. Animal studies show that aromatase inhibition increases levels of proteins involved in blood vessel constriction and sodium retention. Estrogen normally helps keep the receptors that tighten blood vessels in check. Without it, those receptors become more active, and blood pressure rises. Research in rats also found that aromatase inhibition promoted kidney injury, suggesting the blood pressure effects aren’t limited to blood vessels alone but involve changes in how the kidneys handle fluid and salt.
How Quickly It Can Happen
Changes to blood vessel function can begin soon after starting letrozole. The American Heart Association recommends at least weekly blood pressure monitoring during the first 4 to 8 weeks of treatment for patients on cancer drugs known to raise blood pressure, including aromatase inhibitors. Blood pressure can also shift when you stop the medication, so monitoring during that transition matters too.
In one study, women who had been on aromatase inhibitors for a median of about 35 months (roughly three years) had significantly higher systolic blood pressure than controls. This suggests the effect persists with long-term use rather than being a temporary adjustment your body corrects on its own.
Letrozole Compared to Tamoxifen
If you’ve heard that tamoxifen is another option for hormone-positive breast cancer, you may wonder how the two compare on heart health. A large meta-analysis found that aromatase inhibitors carry about 16% higher odds of cardiovascular events compared to tamoxifen. But the picture is more nuanced than it first appears: much of that gap comes from the fact that tamoxifen has its own mild cardioprotective effects rather than aromatase inhibitors being especially dangerous.
A pooled analysis of five clinical trials found the odds of developing hypertension were about 1.3 times higher with aromatase inhibitors than with tamoxifen, though this particular finding didn’t reach statistical significance. The BIG 1-98 trial, one of the largest studies comparing the two drugs, found an increase in serious cardiac events with letrozole specifically. So while the blood pressure risk with letrozole is moderate, it’s consistently higher than what’s seen with tamoxifen.
Long-Term Cardiovascular Concerns
Blood pressure is just one piece of a broader cardiovascular picture. Aromatase inhibitors as a class have been linked to higher rates of high cholesterol and, with prolonged use, increased risk of arrhythmias and acute heart disease. A population-based study found that patients treated with aromatase inhibitors for more than four years had elevated risk for both irregular heart rhythms and ischemic heart disease (the type caused by reduced blood flow to the heart).
That said, a large systematic review could not confirm a statistically significant link between aromatase inhibitors and some individual metabolic risk factors like high blood sugar, insulin resistance, or weight gain. The cardiovascular concern appears to center more on blood vessel stiffness, blood pressure, and cholesterol than on metabolic syndrome overall.
Monitoring Your Blood Pressure at Home
If you’re starting or currently taking letrozole, tracking your blood pressure at home gives you and your care team useful data. A simple automated cuff from a pharmacy works well. Take readings at the same time each day, ideally in the morning before any medications, sitting quietly for a few minutes first. Write them down or use an app so you can spot trends rather than reacting to any single number.
During the first 4 to 8 weeks, checking at least once a week is a reasonable baseline. If your readings start creeping above 130/80 consistently, or if you notice symptoms like headaches, dizziness, or visual changes, that information helps your doctor decide whether to add or adjust blood pressure treatment. Because letrozole can cause blood pressure to fluctuate (not just rise steadily), the timing and frequency of your monitoring may need to be adjusted along the way.
What This Means for Your Treatment
An 8% incidence rate means the large majority of people on letrozole won’t develop clinically significant hypertension. But if you already have borderline or elevated blood pressure, the added effect of estrogen suppression could push you over the line. The risk is real but manageable, and it rarely requires stopping letrozole entirely. Blood pressure medications work just as well when the cause is drug-related, and the benefit of letrozole in reducing breast cancer recurrence is substantial.
The key is not to ignore it. Blood pressure rises silently, and the cardiovascular effects of estrogen loss can compound over the years of treatment that letrozole typically requires. Regular monitoring, especially early on, catches the problem while it’s still easy to address.