Aspirin may help fight aggressive breast cancer by making hard-to-treat tumours more responsive to anti-cancer drugs, doctors say.
A team at the Christie NHS Foundation Trust, in Manchester, are beginning a trial with triple-negative breast-cancer patients.
They suspect it is aspirin's anti-inflammatory properties rather than its analgesic effect that gives the boost.
Animal studies have already shown encouraging results.
There is some evidence aspirin might help prevent certain other cancers and lower the risk of it spreading.
But it is too early to recommend people start taking it. More research is needed.
About 8,000 women are diagnosed with triple-negative breast cancer in the UK each year - a less common but often more aggressive type of breast cancer that disproportionately affects younger women and black women.
These tumours lack receptors some other breast cancers have, meaning certain treatments, such as herceptin, will not work. Although, other medicines and treatments can help.
In the trial, funded by a research programme run by the charity Breast Cancer Now, some patients will be given aspirin as well as immunotherapy drug avelumab before they receive surgery and chemotherapy treatment.
If it is successful, there could be further clinical trials of aspirin and avelumab for incurable secondary triple-negative breast cancer, when cancer cells that started in the breast spread to other parts of the body.
Beth Bramall, 44, from Hampshire was diagnosed with triple-negative breast cancer in 2019.
She said: "There's no easy cancer but triple negative is particularly gruelling, with few treatment options and a long and debilitating treatment plan.
"It floored me, with side-effects of hair loss, nausea, joint and muscle pain, diarrhoea and constipation, burning palms and feet, migraines, night sweats and fatigue like I've never known before.
"I'm blessed that I've had a pathological complete response to treatment.
"But it's been the hardest 18 months for me and my family.
"And I have over two more years of treatments and scans ahead."
Trial lead Dr Anne Armstrong said: "Not all breast cancers respond well to immunotherapy.
"Trialling the use of a drug like aspirin is exciting because it is so widely available and inexpensive to produce.
"We hope our trial will show that, when combined with immunotherapy, aspirin can enhance its effects and may ultimately provide a safe new way to treat breast cancer."
Co-researcher Dr Rebecca Lee said their lab findings suggested that aspirin can make certain types of immunotherapy more effective by preventing the cancer from making substances that weaken the immune response.
"We hope aspirin can dampen down bad inflammation so the immune system can get on with the job of killing cancer cells," she said.