Former President Joe Biden signaled he will have to scale back the tour to promote his upcoming memoir “Promise me, America,” as he continues to undergo treatment for cancer.

“Some days are better than others,” Biden wrote in a lengthy post on X on Sunday. “It hasn’t stopped me. But sometimes it slows me down.”

As a result, Biden said he would no longer “be able to do as many conversations about my book as I’d hoped. However, I’m hopeful the words on the page speak for themselves.”

Months after leaving office in January 2025, Biden, 83, announced that he had been diagnosed with an aggressive form of prostate cancer and has since undertaken courses of radiation therapy and hormone treatment.

Last year, his office also shared that he had undergone surgery to remove cancerous skin cells.

The memoir is due out on Nov. 17 and details his four years in the White House, including “the decisions I made, and why I made them,” he wrote. “More than anything, it’s about my faith in the American people and the great American experiment in self-governance and democracy.”

In August, his son, Hunter Biden, told the BBC in an interview that his father’s cancer had spread to other parts of his body and was causing him pain, even as he continues to speak out on public issues.

“The cancer has spread, metastasized into his bones and further,” Hunter Biden told the BBC in August. “It’s very painful and it’s very debilitating in many respects.”

“It’s really sad to watch,” he added.

Former first lady Jill Biden also previously told NBC News: “I think Joe will live with cancer till the rest of his life.”

Biden was the oldest man to serve as U.S. president and his age and health were a source of concern for much of his time in office.

The memoir of the 46th president delves into “his campaign, his presidency, his decision to run again,” according to the publisher, Hachette Book Group. It also “reveals the deeply agonizing calculation behind his decision in the summer of 2024 to step aside from the presidential race and to put his party and the nation before his personal ambitions.”

Biden suspended that re-election bid and ultimately endorsed his vice president, Kamala Harris, after mounting pressure to step aside. Biden has frequently defended himself against accusations about his mental acuity and memory.

Biden said in the post Sunday that his doctors were “encouraged by how I’ve responded to treatment,” and thanked the public for their prayers and well wishes as he vowed to keep fighting.