Kate Middleton, the Princess of Wales, announced on Friday that she is now undergoing chemotherapy treatment for an undisclosed cancer diagnosis. “In January, I underwent major abdominal surgery in London and at the time, it was thought that my condition was non-cancerous,” she shared on X (formerly known as Twitter). “The surgery was successful. However, tests after the operation found cancer had been present. My medical team therefore advised that I should undergo a course of preventative chemotherapy and I am now in the early stages of that treatment.”

Back in January, Kensington Palace revealed that the Princess of Wales was undergoing a planned abdominal surgery and would remain in the hospital for 10 to 14 days before returning home to recover. They stated that she was unlikely to return to her royal duties until after Easter. Updates, they said, would only be shared if there was “significant new information.” Rumors have abounded since.

People on the internet soon speculated about potential other reasons for Kate’s absence from her royal duties, including that her health issues were more serious than the statement suggested.

The Princess of Wales isn’t the only royal to receive a cancer diagnosis in recent weeks. King Charles, her father-in-law, also has a non-specified type of cancer, which Buckingham Palace announced in February, following treatment for an enlarged prostate.

A lot remains unknown about the princess’s specific diagnosis and treatment plan. In the meantime, here’s what to know about preventative chemotherapy.

What is preventative chemotherapy?

Chemotherapy is generally used preventatively to keep cancer from coming back after being successfully treated with surgery or radiotherapy. Doctors call this “adjuvant therapy.”

When someone is prescribed preventative chemotherapy, this typically suggests that there was no visible cancer left behind after surgical removal or radiation treatment and no evidence that tumors had spread to other parts of the body — in other words, that the cancer was not metastatic.

However, even if a cancerous tumor has been completely removed or destroyed, cancer cells can break off from primary tumors and travel to other parts of the body, and cells too small for doctors to see may still linger. A course of preventative chemotherapy — which may come in the form of pills or an IV — helps kill off any of these stray cancer cells before they can start to multiply, according to the Mayo Clinic.

Chemotherapy isn’t the only way to to do this, but it is one of the most effective and commonly used. Other preventative treatments include hormone therapies that can cut off the fuel supply to tumors that are driven by hormones (such as certain types of breast cancer), radiation therapy, immunotherapy and targeted therapies, which block non-hormonal drivers of cancer.

What are the side effects of preventative chemotherapy?

Preventative chemotherapy is not necessarily given at a lower dose than a typical course used to treat existing cancer. So the potential side effects are the same, although they vary depending on the specific type of cancer the treatment targets and the medication used. In general, side effects can include:

How long do people have to be on preventative chemotherapy?

According to the Mayo Clinic, treatment may last only a few weeks, or continue as long as 10 years.

Whether and how long someone undergoes preventative chemotherapy and how effective the therapy will be depends on the type of cancer they’re being treated for. The treatment has been effective for preventing recurrences of both breast and colon cancer, for example.

But its effectiveness also depends on the stage of the cancer, whether the disease is hormone-dependent and whether it’s spread to any lymph nodes. In some cases, and for certain cancers, doctors don’t recommend preventative chemotherapy because it may do little to prevent the disease from returning, while making the patient’s quality of life much poorer.