Kate Middleton is “not out of the woods yet” after her cancer diagnosis and will be in treatment for a few more months, she shared on Instagram.

“I am making good progress, but as anyone going through chemotherapy will know, there are good days and bad days,” the Princess of Wales wrote. “On those bad days you feel weak, tired and you have to give in to your body resting. But on the good days, when you feel stronger, you want to make the most of feeling well.”

The 42-year-old mother of three who also released a new picture of herself and is scheduled to attend the King’s Birthday Parade, her first public event in months this weekend.

In March, she shared in a statement that she underwent major abdominal surgery in London, which was deemed 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,” she wrote.

Here’s what to know about preventative chemotherapy.

What is preventative chemotherapy?

“Preventative chemotherapy isn’t so much a technical term — it’s a lay term — and it’s more akin to adjuvant treatment, meaning ‘additional,'” Dr. Ginger Gardner, a gynecological oncologist at Memorial Sloane Kettering Cancer Center and chair of the Foundation for Women’s Cancer, tells Yahoo Life. “That generic term might be applied to a situation in which a tumor was removed and yet [doctors want to] provide some treatment coverage” to prevent the cancer from coming back. It could also be used to prevent recurrence after a tumor has been destroyed by radiation 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 — might be deployed to “reduce the risk of any microscopic cells returning or delaying their opportunity to do so,” Gardner says.

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. However, adds Gardner, “cancer is not just one disease,” meaning that the type, duration and dose of chemotherapy is carefully chosen based on the type of cancer, its stage and its biology, as well as what’s best for the patient’s quality of life.

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. Middleton said in March that she was “in the early stages” of treatment, and still had a few months ahead as of June 14.

The effectiveness of chemotherapy when used this way 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 at all.

“If there’s enough benefit of adjuvant therapy to retain durable remission, then it’s worth some side effects,” Gardner says. “But if the disease’s biology means that someone is not going to benefit from this type of therapy, then it’s not worth it and it’s in that balance that we make decisions about adjuvant treatment.”

In the Princess of Wales’s case, “Whatever this is, the fact that she took proactive measures about something that was supposedly benign and, from what she said, has achieved disease clearance and is taking proactive steps [to prevent recurrence] and being thoughtful about her children, that’s important,” Gardner says.