King Charles III was briefly hospitalized for observation on Thursday after experiencing “temporary side effects” related to a scheduled cancer treatment, Buckingham Palace said in a statement.

His engagements on Thursday afternoon and Friday were cancelled.

“His Majesty has now returned to Clarence House and, as a precaution, acting on medical advice, tomorrow’s daily programme will also be rescheduled,” the palace said. “I would like to apologise to anyone who may have been inconvenienced or disappointed by this.”

The monarch’s health has been closely monitored since the monarch announced early last year that he had an undisclosed form of cancer. Charles, 76, withdrew from public duties for about three months but continued to carry out official duties such as reviewing government documents and meeting with the prime minister.

Charles’ cancer diagnosis has put a strain on the British monarchy, which is still evolving after the late Queen Elizabeth II’s 70-year reign.

When he succeeded his mother in September 2022, Charles’s task was to show that the 1,000-year-old institution was relevant in a modern nation whose citizens come from all corners of the world. But the task requires a lot of time and energy.

While the duties of a constitutional monarch are largely ceremonial, the royal whirlwind can be exhausting. In addition to the occasional parade in full royal regalia, there are meetings with political leaders, dedication ceremonies, and events honoring the achievements of British citizens. Charles spent 161 days on royal duties in his first year on the throne.

Charles’s illness came as his niece, Kate, Princess of Wales, was also diagnosed with cancer. Kate, the wife of Prince William, took more than six months off before returning to public duties in late September.