The Prince of Wales questioned if his Duchy Originals beer had caused an 83-year-old to faint when the man collapsed as Charles made a speech at Brewers' Hall.
During a visit, where the prince was being made an honorary Liveryman, Roddy Pryor, a past master of the Livery, fainted.
Charles interrupted his speech, saying "Oh dear, is that the effects of Duchy beer?", as the brand he established was being served at the event.
Mr Pryor, who was master of Brewers' Company in 1980, cut his eye and forehead as he fell.
Paramedics attended the scene and Charles asked how Mr Pryor was, adding: "I hope he's all right."
Mr Pryor's faint was attributed to "standing for too long" and "the heat of the room", a representative of the Brewers' Company told the Press Association.
"He's absolutely fine. He was given a clean bill of health by the paramedics and walked downstairs, getting his pre-booked taxi home like any other guest.
"It was the heat of the room and he had been standing for too long," he said, adding that Mr Pryor had said he was "too stubborn" to sit down.
The ceremony in the City of London took place in front of 20 other Liverymen and guests.
During his acceptance speech, Charles spoke of his Pub is the Hub initiative which offers advice and support to licensees and owners who wish to offer community services from their pub.
The prince said: "I'm so worried about a loss of rural pubs.
"The great thing was to try and ensure that we could at least keep some of these places going by finding new uses...so that's what we've managed to do."
Expressing his gratitude for being made a Liveryman, he said: "Nothing could make me more proud if I may say so. Quite why you wanted to, I don't know - whatever it is, I'm deeply touched."
Current Master of the Livery Miles Jenner said he was "tremendously honoured" by the prince's visit.
The Brewers' Company is one of the oldest Livery Companies in the City of London, established by a Royal Charter granted by Henry VI in 1438.
The prince was shown the charter as part of his visit.
Charles has been involved in the brewing industry for many years and, in 2002, the Prince was named Beer Drinker of the Year by the All Party Parliamentary Beer Group.