King Charles health update: A look at the possible side effect the monarch had from cancer treatment

King Charles has been seen in public for the first time after spending time in hospital after experiencing side effects from cancer treatment. The monarch, 76, was seen leaving Clarence House in London in a car on Friday morning (March 28) and waved at well-wishes.

In a statement, Buckingham Palace said: “Following scheduled and ongoing medical treatment for cancer this morning, the King experienced temporary side effects that required a short period of observation in hospital. His Majesty’s afternoon engagements were therefore postponed.

“His Majesty has now returned to Clarence House and as a precautionary measure, acting on medical advice, tomorrow’s diary programme will also be rescheduled. His Majesty would like to send his apologies to all those who may be inconvenienced or disappointed as a result.”

In February 2024, it was revealed that King Charles had been diagnosed with an undisclosed form of cancer and would start treatment. In April of last year, the King returned to public duties.

In December 2024, it was revealed that King Charles would continue with his cancer treatment in 2025 but palace sources said that “his treatment has been moving in a positive direction and as a managed condition the treatment cycle will continue into next year.”

Three days ago, it was revealed that King Charles and Queen Camilla’s state visit to the Vatican had been postponed. However, the reason for the postponement had nothing to do with King Charles’s cancer treatment but because the Pope needed to rest after only recently being discharged from hospital.

Pope Francis, 88, was admitted to hospital in Rome in February after suffering breathing difficulties, he was diagnosed with pneumonia in both of his lungs and was considered to have been in a “critical” condition. According to a statement released by Buckingham Palace earlier this year, the King and Queen’s visit to the Vatican was postponed by “mutual agreement.”

What side effect could King Charles have suffered from his cancer treatment?

Professor Robert Thomas said: “His admission this time is not to do with cancer progression but a side effect of treatment - this is most likely to be a urinary tract infection which is a common side effect of repeated catheterisations - causing a high temperature, cystitis and generally feeling unwell.”

In February 2025, Professor Robert Thomas, who is a full-time NHS Consultant Oncologist at Addenbrooke's and Bedford Hospitals, suggested that “Although it has not been officially announced by "precisely what cancer King Charles' has been diagnosed with, it is very likely to be bladder cancer.”

Professor Robert Thomas also thought that the announcement informing the public that the treatment would be extending well into 2025 “tells us more about the precise type he has - with some concern. From the course of events early in 2024, it appeared that his bladder cancer was picked up by chance, after the biopsy of his prostate for a separate condition called Benign Prostatic Hyperplasia (BPH).

“This was very fortunate as his bladder cancer could have grown for many months without being noticed, at a more advanced stage. The usual first sign of bladder cancer is blood in the urine (Haematuria). It is well established that the earlier the cancers are picked the better the prognosis especially whether the cancer has grown into the muscle bladder wall. The treatments for bladder cancers which have or have not spread into the muscle wall are significantly different.”

Professor Robert Thomas also highlighted the importance of dietary and lifestyle strategies as well as medical and surgical treatments and said that “the factors which help the prognosis include quitting smoking (if relevant), improving gut health, stopping late night snacking and high processed sugar intake, ensuring adequate vitamin D levels, avoiding dietary and environmental chemicals, exercising at least 3 hours a week, trying to get a good night's sleep, avoiding ultra-processed foods, and boosting the intake of phytochemical rich foods.”