"He has to be moved in the coming 24 to 48 hours – these conditions are not good for him," Lamya Essemlali, the head of the Sea Shepherd France marine conservation group, told AFP on Sunday.
Along with the injection, experts have considered the possibility of opening up the locks and leading the whale 100 miles (161 kilometres) back out into the open sea.
The locks were closed at the request of Sea Shepherd, which has been tracking the mammal, in order to prevent it from swimming further inland. But the longer it stays in stagnant freshwater, the less likely it is that it will be able to make the long and exhausting journey.
"We are all doubtful about its own ability to return to the sea," said Essemlali. "Even if we drove it with a boat, that would be extremely dangerous, if not impossible."
On Sunday, Sea Shepherd France issued an update on Twitter, saying that although the severely underweight whale continues to reject food, it is alert and dynamic, and that therefore euthanasia would be premature at this point.
On Saturday, the 13ft whale was seen swimming "calmly" back and forth in the basin between two locks in the Seine. As well as being worryingly lean, spots on the whale's skin also suggest illness.
Isabelle Dorliat-Pouzet, the secretary general of the Eure prefecture in northwestern France, ruled out a direct intervention and extracting the whale from the waters, saying it was too weak to survive an aggressive rescue mission.
Dorliat-Pouzet added that decisions would be made in the best interest of the mammal, but nothing had yet been confirmed.
In May, a sick orca became separated from its pod and was spotted swimming in the river between Le Havre and Honfleur. The orca later died, with the immediate cause found to have been starvation.
Essemlali said one possible explanation for whales straying far from home and into the Seine could be the rise of noise pollution generated by human activity.i
