tisane

yrttijuoma A medicinal drink, originally made from barley soaked in water; a herbal tea. (defdate)
1831, Alexander Macaulay, http://books.google.co.uk/books?id=lGZZAAAAIAAJ A dictionary of medicine, designed for popular use, 2nd edition, page 454:

Ptisan. A diluent drink which makes a great figure in the dietetic precepts of the ancients.

“Neither,” said Poirot, “I shall go to bed and take a tisane. The expected has happened ….”

The sick people would take away also some herbs for their ptisan, some wine and other comforts ….

As soon as he had opened the door he worked his way back to his high-backed Queen Anne armchair, where he picked up his bone-china cup and took a sip of a rarefied tisane.