The pain returns each month, sometimes lingering after a period ends. Even when treatment brings relief, it may not stay away. For many women living with endometriosis, this cycle continues for years.

Scientists searching for a way to ease that pain have turned to an unexpected drug: niclosamide, used to kill intestinal parasites called tapeworms.

In a new study published in Advanced Healthcare Materials, a version of the drug attached to a specially designed carrier reduced endometriosis-associated growths and pain-related sensitivity in mice.

Earlier research had offered clues that niclosamide might help. But the researchers faced a challenge: how could they deliver it to cells implicated in sustaining the disease?

Endometriosis occurs when tissue similar to the lining of the uterus grows outside it.

Affecting roughly one in 10 women of reproductive age, it can cause inflammation, severe pelvic pain, painful periods, and fertility problems.

Current treatments include hormonal therapies and surgery, but symptoms can return, and there is currently no cure.

“Endometriosis is a chronic disease,” Kanako Hayashi, a professor studying reproductive biology at Washington State University in the US, told ScienceAlert.

“Patients suffer from the symptoms (such as chronic pelvic pain) for multiple decades.”

Hayashi and her colleagues are investigating how immune cells contribute to this persistent disease. Previous research has linked endometriosis to changes in immune function, although whether those changes cause the disease or result from it remains unclear.

The researchers focused on immune cells called macrophages. Earlier research discussed in Communications Biology suggests these cells can release inflammatory signals and growth factors that support the abnormal tissue growths, known as lesions.

Previous studies had identified a particular group of macrophages elevated in endometriosis lesions in humans and mice. These cells carry a surface protein called folate receptor beta, or FRβ, which offered a potential target for delivering the drug.

That protein offered a potential target. A carrier capable of binding to it could guide niclosamide into these macrophages.

Researchers from Washington State University, the University of Michigan, and the University of Florida designed a small, branching molecular structure called a dendrimer. They attached folic acid, which binds to FRβ, and linked niclosamide to the carrier through a chemical connection designed to release the drug inside cells.

Diagram showing a dendrimer carrying niclosamide into FRβ-positive macrophages associated with endometriosis.
The experimental approach uses a molecular carrier to deliver niclosamide to specific immune cells associated with endometriosis. It has been tested in mice. (Image supplied by Kanako Hayashi)

The aim was to improve the drug’s water solubility while directing it toward particular cells.

Niclosamide dissolves poorly in water, and relatively little reaches the bloodstream when swallowed. Although earlier mouse research showed promising effects against endometriosis, those properties complicate its development as a long-term treatment.

Hayashi explains that taking a drug every day for a chronic condition can be difficult to sustain and raises concerns about potential side effects. The new carrier is intended to deliver niclosamide to the targeted cells while limiting unwanted effects elsewhere.

To investigate the macrophage target, the researchers analyzed 10 previously published human cell datasets alongside newly collected cells from fluid samples from the abdominal cavities of four patients.

Fluorescence images showing dendrimer distribution in mice and uptake by FRβ-positive macrophages in lesions and abdominal cavity cells.
Fluorescence imaging tracks the experimental carrier in mice and shows its uptake by targeted immune cells. Blue marks cell nuclei, green marks FRβ, and red marks the labeled carrier. (Hayashi, Sharma et al., Advanced Healthcare Materials, 2026)

The experimental treatment itself was tested in mice. Researchers induced endometriosis-like lesions by injecting uterine lining tissue from donor mice into the animals’ abdominal cavities.

Seven days later, they gave the animals one treatment injection. Controls received the carrier alone; other groups received the carrier linked to niclosamide at two different doses.

Two weeks after treatment, mice receiving the drug had fewer and smaller lesions than controls. Four of eight mice in the lower-dose group and five of eight in the higher-dose group had no detectable lesions at that assessment.

The team also applied pressure to the animals’ abdomens and hind paws using fine filaments, measuring their withdrawal responses. Both doses eased heightened pain-related sensitivity.

These measurements show improved responses in mice. They cannot establish how much pain relief a person with endometriosis would experience.

The treatment groups assessed at this point were small, with eight mice each, and follow-up extended only two weeks after dosing. The study did not establish long-term benefits, the safety of repeated treatment, or effectiveness in humans.

“Our study is still in preclinical stages,” Hayashi says.

She explains that several steps remain before the team can obtain approval to conduct clinical trials. “It is still a long way from being commercialized.”

Since publication, patients have contacted the team asking to join or be listed for future trials. Hayashi says people with endometriosis are desperate for new therapies.

Several authors, including Hayashi and Anjali Sharma, disclose pending patents and invention disclosures covering the dendrimer technology.

The research has been published in Advanced Healthcare Materials.