New Low-Dose Breast Density Treatment Shows Promise for Cancer Prevention With Fewer Side Effects

Nearly half of all women aged 40 and older have dense breast tissue, a condition that not only makes mammograms harder to read but also raises the risk of developing breast cancer. Now, a new clinical trial suggests that a low-dose medication called (Z)-endoxifen may offer a safer, more tolerable way to reduce breast density—and potentially lower cancer risk—without the harsh side effects that often cause patients to quit standard treatments.

Published in the Journal of the National Cancer Institute on April 27, the study tested two daily doses of (Z)-endoxifen, a natural byproduct of the well-known breast cancer drug tamoxifen. Researchers wanted to see if this compound could safely shrink dense breast tissue while causing fewer unpleasant symptoms like hot flashes, night sweats, and fatigue.

If you or someone you know has dense breasts—or has struggled with the side effects of tamoxifen—this research could signal a major shift in how doctors approach breast cancer prevention. Here is a closer look at what the study found, what it means for patients, and what experts say you should keep in mind.

What Are Dense Breasts and Why Do They Matter?

Dense breasts are a common condition where the breast has more glandular and fibrous tissue than fatty tissue. This is not something you can feel—it is only visible on a mammogram. But dense tissue matters for two key reasons.

First, dense tissue can hide tumors on a mammogram, making breast cancer harder to detect early. Second, having dense breasts is itself an independent risk factor for breast cancer. Women with extremely dense tissue are four to six times more likely to develop breast cancer than women with mostly fatty breasts.

Doctors have known for years that lowering breast density could reduce cancer risk. Tamoxifen, a drug used for both treatment and prevention, can reduce breast density by about 20% to 30%. But tamoxifen comes with a long list of side effects—including hot flashes, blood clots, and uterine cancer—that cause many women to stop taking it.

This is where (Z)-endoxifen enters the picture.

How the KARISMA Endoxifen Trial Worked

The KARISMA Endoxifen trial was a carefully designed Phase II study—meaning it tested the drug in a small group of people to see if it worked and was safe. The study was double-blind, randomized, and placebo-controlled, which is the gold standard for medical research. Neither the participants nor the researchers knew who was getting the real drug until the study ended.

Researchers enrolled 240 healthy premenopausal women between the ages of 40 and 55. All participants were recruited from Sweden’s national breast cancer screening program between December 2021 and November 2023. To qualify, women had to have regular menstrual cycles (or confirm they were premenopausal through blood tests) and a baseline mammogram showing measurable breast density. Women taking medications that could interfere with how endoxifen is processed in the body were excluded.

Participants were split into three groups:

• One group received a placebo (a dummy pill)
• A second group took 1 milligram (mg) of (Z)-endoxifen daily
• A third group took 2 mg of (Z)-endoxifen daily

All treatments were given as oral capsules for six months. Mammograms were taken at the start of the study, at three months, at six months, or whenever a participant stopped early. A specialized computer program called STRATUS measured breast density in square centimeters to ensure accuracy.

Researchers also tracked side effects using a digital app and a validated symptom questionnaire called the Breast Cancer Prevention Trial Eight Symptom Scale (BESS Plus). They monitored vital signs, blood chemistry, and any participant-reported problems throughout the trial.

The Main Finding: Significant Reductions in Breast Density

The results were clear: both doses of (Z)-endoxifen reduced breast density far more than the placebo.

– Women taking 1 mg of (Z)-endoxifen saw a 19.3% reduction in breast density

• Women taking 2 mg saw a 26.5% reduction
• The placebo group showed virtually no change

These reductions are similar to what doctors typically see with the standard 20 mg dose of tamoxifen—but at just one-tenth or one-twentieth of the dose. That is a dramatic difference.

Blood tests confirmed that the drug was reaching the bloodstream as expected. Average levels were 4.75 ng/mL in the 1 mg group and 9.69 ng/mL in the 2 mg group. Interestingly, breast density reduction seemed to plateau at concentrations of 3 to 4 ng/mL, suggesting that higher doses may not provide extra benefit.

Side Effects: Lower Dose, Fewer Problems

One of the biggest hurdles with tamoxifen is its side effects. Many women stop taking it within the first year because they cannot tolerate the hot flashes, night sweats, fatigue, and other symptoms.

In this trial, the overall number of side effects reported was similar across all groups. But women taking (Z)-endoxifen did report more symptoms linked to the drug itself. The most common side effects included:

• Hot flashes and night sweats (vasomotor symptoms)
• Fatigue
• Diarrhea
• Vaginal bleeding
• Vision disturbances
• Breast tenderness
• Dry mouth
• Weight loss

However, the 1 mg dose stood out. Women in this group reported significantly fewer vasomotor symptoms than those on the 2 mg dose. In fact, the 1 mg group actually reported fewer problems with diarrhea, vaginal bleeding, vision issues, breast tenderness, dry mouth, and weight loss compared to the placebo group. That is unusual and suggests the lower dose may have a very tolerable side-effect profile.

Fewer women on the 1 mg dose dropped out of the study due to side effects compared to those on the 2 mg dose. No serious changes were seen in blood chemistry, hematology, or vital signs, and any serious adverse events were rare and not related to the study drug.

What Experts Say About the Results

Dr. Blen Tesfu, a physician and medical advisor at Welzo who was not involved in the trial, called the findings important. She noted that the study shows a much lower dose of (Z)-endoxifen can be just as effective as the standard tamoxifen dose at reducing breast density—a key marker for breast cancer risk.

“Since there are established associations between breast density and breast cancer risk, even modest reductions could have implications for preventive approaches,” Tesfu told Healthline.

She also highlighted the potential for better patient compliance. Many women stop taking tamoxifen because of side effects, leaving them without adequate hormonal prevention.

“This could help to address what has been identified as one of the primary impediments to patient compliance with long-term use of hormone-based drugs,” she said.

Brian Clark, a certified registered nurse anesthetist and CEO of United Medical Education, agreed. He pointed out that many people who cannot tolerate tamoxifen’s side effects simply miss out on effective prevention altogether.

“This opening of the population to a drug that offers similar effects at 1 mg opens the door to populations previously not afforded this quality of care,” Clark told Healthline. He added that this could change how breast cancer risk reduction is approached across entire populations.

Important Caveats: What This Study Does Not Yet Prove

It is important to understand that this was a proof-of-concept trial. That means it was designed to show whether the drug works and is safe, but it is not the final word.

Larger, longer studies will be needed to confirm that (Z)-endoxifen actually reduces breast cancer risk—not just breast density. While breast density is a well-known marker for risk, it is not the same as proving that fewer cancers occur.

Also, the trial only included premenopausal women. It is not yet clear how the drug would work in postmenopausal women, who make up the majority of breast cancer cases.

Finally, the study was conducted in Sweden, and participants were recruited from a national screening program. Results may vary in more diverse populations with different genetic backgrounds, diets, and lifestyles.

Practical Takeaways for Readers

If you have dense breasts or are considering breast cancer prevention options, here is what you should know right now:

– Dense breasts are common and important. About half of women over 40 have them. If you do not know your breast density, ask your doctor after your next mammogram.

• Tamoxifen works but is hard to tolerate. Many women stop taking it due to side effects. This new approach could offer a gentler alternative.
• (Z)-endoxifen is not yet available. This is still experimental. It is not approved by the FDA or available at pharmacies. Do not try to obtain it on your own.
• Talk to your doctor about clinical trials. If you are interested in participating in research on breast density reduction, ask your healthcare provider if any trials are enrolling near you.
• Lifestyle matters too. While medication can help, maintaining a healthy weight, limiting alcohol, and staying physically active are also linked to lower breast cancer risk.

The Bottom Line

The KARISMA Endoxifen trial offers encouraging evidence that a low dose of (Z)-endoxifen can reduce breast density as effectively as tamoxifen—but with fewer and milder side effects. For the millions of women who have dense breasts and worry about breast cancer, this could eventually mean a prevention option that is easier to stick with over the long term.

Still, experts caution that more research is needed before this becomes a standard treatment. If the results hold up in larger trials, (Z)-endoxifen could change the landscape of breast cancer prevention—making it more accessible, more tolerable, and ultimately more effective for the people who need it most.

Source: Healthline