The monarch, 76, had to cancel his engagements for yesterday afternoon and today.
But an aide described the setback as a "minor bump in the road" and a royal source said: "The King will want to get back on the horse as soon as possible."
Charles, who was diagnosed with cancer last February, returned home to Clarence House last night.
The King made the journey to The London Clinic in Marylebone by car and a palace aide insisted Charles' treatment was still "very much headed in the right direction".
Royal sources said the King was not joined by the Queen during his brief stay.
Charles, who has been undergoing weekly treatment for cancer since his diagnosis last year, had to cancel an away day in Birmingham today, where he was scheduled to meet local communities.
It is understood the side effects, of which specifics have not been disclosed, were temporary and not uncommon with many medical treatments.
A spokesman said the King was greatly disappointed to be missing the engagements and every effort would be made to reschedule at a later date.
The King is due to make a historic state visit to Italy in 10 days, having returned to full-time dutie...