L'Hotel-Dieu hospital in Paris

Six members of the medical team at the Hôtel-Dieu hospital in Paris fell into a deep sleep Tuesday after their coffee was spiked with a powerful anti-psychotic drug, French daily Le Parisien reports.

Four of them were still in a deep sleep Tuesday evening but latest reports confirm they were all awake just before lunchtime Wednesday.

Their lives were not in danger but police have opened a criminal enquiry for attempted poisoning.

Doctors, nurses and auxiliary carers heading to the staff room for a routine pick-me-up experienced quite the opposite, after an unknown person laced the coffee thermos with a high dose of Loxapac.

Staff complained of nausea and were found sound asleep by midday.

A single 60 ml dose of Loxapac can make 15 90 kg people sleep for half a day.

Investigators believe that the incident may just be a prank,  but a source near to the enquiry told Le Parisien newspaper that a joke, even in very bad taste, does not make sense in the context of this drug.

Police are currently trying to trace a mother who was treated for psychiatric problems on Monday night and had coffee in the staff room before leaving the hospital on Tuesday morning.

Preliminary results of the inquiry confirm that the staff room is never locked, despite the fact that use is reserved for hospital personnel.