Anastrozole, a medication prescribed to treat hormone-receptor-positive breast cancer in postmenopausal women, causes side effects in most people who take it. The most commonly reported issues are joint pain, hot flashes, fatigue, and mood changes. Some effects are mild and manageable, while others, particularly bone loss and cardiovascular changes, develop gradually and require monitoring over the course of treatment.
Joint and Muscle Pain
Joint pain is the side effect most likely to interfere with daily life. It occurs because anastrozole works by blocking the body’s production of estrogen, and estrogen plays a protective role in joints, bones, and the tissues surrounding them. When estrogen levels drop sharply, joints lose some of their natural cushioning and lubrication. The result is stiffness, aching, and sometimes swelling, particularly in the hands, wrists, knees, and hips. Many women describe it as feeling decades older overnight.
A systematic review of the biological mechanisms found that estrogen deprivation affects immune function, bone remodeling, and pain sensitivity all at once, which explains why the discomfort can feel widespread rather than isolated to one joint. The pain typically appears within the first few months of starting the medication and, for some women, becomes the primary reason they consider stopping treatment early.
There is some evidence that omega-3 fatty acid supplements can help, though the benefit appears to depend on body weight. Research from Columbia University found that in women with a BMI of 30 or higher, omega-3 supplements reduced joint pain by nearly three points on a ten-point scale, roughly double the improvement seen with a placebo. For women who were not obese, omega-3s didn’t outperform the placebo. Acupuncture and regular exercise have also shown promise for reducing treatment-related joint pain, and side effects from omega-3 supplements are generally limited to mild reflux or a fishy aftertaste.
Bone Density Loss and Fracture Risk
Because estrogen helps maintain bone strength, long-term anastrozole use causes measurable bone thinning. Data from the large ATAC trial, which followed over 9,000 women, showed that after five years of treatment, women on anastrozole lost a median of 6.08% of bone density in the lower spine and 7.24% in the hip. For comparison, women in the same trial who took tamoxifen actually gained a small amount of bone density in both areas.
This bone loss translates directly into more fractures. In the ATAC trial, 11% of women on anastrozole experienced a fracture during treatment, compared to 7.7% of women on tamoxifen. That’s roughly a 49% higher odds of fracture. Your doctor will likely recommend a bone density scan before starting anastrozole and periodically during treatment. Weight-bearing exercise, calcium, and vitamin D are standard strategies to slow bone loss while on the medication.
Hot Flashes and Menopausal Symptoms
Hot flashes are one of the earliest and most frequent complaints. They happen for the same reason as menopausal hot flashes: a sudden drop in estrogen disrupts the body’s temperature regulation. Women who had already gone through menopause and thought they were past hot flashes often find them returning. Night sweats, which can interrupt sleep and compound fatigue, are closely related. Vaginal dryness and decreased libido also fall into this category of estrogen-withdrawal symptoms.
Cholesterol and Heart Health
Anastrozole’s effect on cholesterol is nuanced. A meta-analysis of randomized controlled trials found that in the first three months, the drug actually lowered total cholesterol slightly (by about 2.7 mg/dL). However, with longer use beyond three months, HDL cholesterol (the protective kind) dropped by about 1.7 mg/dL. The drug did not appear to affect triglycerides or LDL cholesterol.
A small drop in HDL may not sound alarming on its own, but over five years of treatment it can contribute to a gradual shift in cardiovascular risk profile. This is especially relevant for postmenopausal women, who already face rising heart disease risk as natural estrogen declines. Periodic cholesterol checks during treatment help catch any meaningful changes early.
Cognitive Changes
Many women on anastrozole report difficulty with memory, concentration, and mental clarity, often described as “brain fog.” This isn’t just anecdotal. A prospective study from the University of Pittsburgh found that postmenopausal women taking anastrozole had significantly poorer cognitive function compared to healthy postmenopausal women. More concerning, their cognitive performance in areas like attention and memory continued to decline for at least 18 months after starting the drug, suggesting the effect worsens with longer use rather than leveling off.
This can show up as forgetting words mid-sentence, struggling to follow complex conversations, or having trouble with tasks that previously felt automatic. For women who are still working or managing demanding schedules, these changes can be genuinely disruptive. The cognitive effects appear to be linked to estrogen deprivation in brain regions involved in memory and executive function.
Mood Changes and Depression
Estrogen influences mood-regulating brain chemistry, so its suppression can trigger irritability, anxiety, and depression. Some women experience these as a general emotional flatness, while others notice more pronounced mood swings. These psychological effects overlap with the fatigue and sleep disruption caused by night sweats, creating a cycle where poor sleep worsens mood, and low mood makes it harder to cope with physical symptoms. If you notice a significant shift in your emotional well-being after starting anastrozole, that’s a recognized effect of the drug, not something to dismiss.
Fatigue
Persistent tiredness is one of the most common complaints and one of the hardest to pin down, since it can stem from multiple sources at once: the drug itself, disrupted sleep from night sweats, the emotional toll of cancer treatment, or the cumulative effect of joint pain making everyday movement harder. Unlike the fatigue that lifts after a good night’s rest, anastrozole-related fatigue tends to be a low-grade constant. Regular physical activity, even moderate walking, has been shown to help, though it can feel counterintuitive when you’re already exhausted.
Rare but Serious Reactions
Severe side effects are uncommon but worth knowing about. Anastrozole can, in rare cases, trigger serious allergic reactions including anaphylaxis (a whole-body allergic response) and angioedema (rapid swelling of the face, lips, tongue, throat, hands, or feet). Skin reactions are also possible, including blistering, peeling, or red lesions with a purple center. Sores or white spots in the mouth or on the lips have been reported as well. These reactions require immediate medical attention.
Drug Interactions to Know About
Because anastrozole works by eliminating estrogen, any product that contains or boosts estrogen will directly undermine its effectiveness. This includes hormone replacement therapy, certain vaginal creams, and some over-the-counter supplements. Beyond estrogen products, certain medications require caution when combined with anastrozole, including some antipsychotics, specific heart rhythm medications, and certain pain medications including hydrocodone and methadone. A full review of all medications and supplements you’re taking, including anything you consider routine, is important before starting treatment.
How Long Side Effects Last
Most side effects are present throughout the course of treatment, which typically lasts five years (and sometimes extends to ten for higher-risk cancers). Joint pain and hot flashes tend to peak in the first year and may gradually become more tolerable, though they rarely disappear entirely. Bone density loss is cumulative, meaning it progresses the longer you take the drug, which is why ongoing monitoring matters. Cognitive effects, based on available research, appear to worsen over at least the first 18 months. After stopping anastrozole, many side effects improve over weeks to months as estrogen levels begin to recover, though bone density lost during treatment may not fully return without active intervention.