The Brideshead Revisited star needed hospital treatment after drinking several litres - leading to a salt imbalance.
Andrews, 55, is now resting and planning to return to the stage next week for his role as Professor Higgins in the West End musical My Fair Lady.
He has missed the production at the Theatre Royal Drury Lane while being treated at a private clinic following the incident on Saturday.
It seems to have been a combination of the spell of hot weather, the fact he was drinking so much water and perspiring as well as performing so often
Penny Wesson, agent
The condition, known as hyponatraemia, is caused by the dilution of sodium in the body. It has similar symptoms to dehydration, such as headaches, nausea and cramp.
Andrews' agent, Penny Wesson, said: "It does affect you if you drink too much and perspire without replacing the salt in your system.
"He's out of the clinic now and we expect him to be back in the show in the next few days."
She added: "He's generally in very good health - you would have to be to do eight performances a week.
"It seems to have been a combination of the spell of hot weather, the fact he was drinking so much water and perspiring as well as performing so often." >
BBC
