"Amnesty International is concerned about the possible deterioration in the health conditions of the striking prisoners," the London-based group said.
The common law detainees, held in the Jaw prison, are protesting against "ill-treatment and lack of access to lawyers and human rights organisations," it added.
"Medical treatment should be immediately provided to the prisoners, especially those with deteriorating health condition ... Prisoners should have immediate access to lawyers and human rights organisations."
Amnesty said there were reports that one hunger striker, named as Yasir Makki, had died in prison earlier this year after authorities failed to provide medical treatment in time.
The number two of the Bahrain Centre for Human Rights, Nabil Rajab, said last Saturday that his organisation had asked the interior ministry to allow BCHR members to visit the prison, but to no avail.
At the time, one prisoner, reached via mobile phone, told Agence France Presse a dozen strikers were gravely ill.
Detainees in Jaw prison have already staged two hunger strikes this year, in January and April, to demand better conditions.