The tablets help patients over the age of 16 to “drift off to sleep” by “blocking the action of histamine” with 25mg of promethazine hydrochloride, it said.

People should take “one or two tablets before bed”, but not for more than seven consecutive days without seeking medical advice, Sanofi said.

Promethazine has been used as a sedative for “over 50 years”, the manufacturer continued.

The retail price for one over-the-counter pack of 14 tablets of Phenergan Night Time 25mg is £4.79.

For more information, contact Sanofi Medical Information on 0845 372 7101