An investigation is under way into how Soham murderer Ian Huntley was able to take an overdose of prescription drugs.
Huntley also took overdoses in 2003 and 2006 while in custody
Huntley, 33, has been released from hospital and returned to HMP Wakefield, where he is serving a life sentence.
The incident on Friday was the third occasion on which Huntley has managed to get hold of enough drugs to take an overdose.
Ex-school caretaker Huntley was found guilty of the 2002 murders of Jessica Chapman and Holly Wells, both 10.
A Prison Service spokeswoman said an investigation was under way into events at the prison on Friday afternoon.
Huntley's condition was not thought to be life-threatening. It is understood he did not lose consciousness and presented himself to prison staff.
He was taken to Pinderfields hospital in West Yorkshire for treatment where he was deemed well enough to return to jail.
Huntley is normally held in the healthcare section of the high-security jail, which is not the same as segregation.
Editor of the Prisons Handbook, Mark Leech, said: "This is a dreadful story for the Prison Service and a huge embarrassment for them, particularly as Huntley is the highest possible security category.
"It is going to require some heads to be banged together to sort this out - they really must get to grips with it because this is not the first time, by any means, that Huntley has tried to take his own life."
'Easy way out'
John Powley, county councillor for Soham and governor at the school where Huntley was employed, said: "Ian Huntley must not be allowed to cheat justice.
"He must not be allowed to take the easy way out because that's what suicide would be."
But Brian Caton, general secretary of the Prison Officers Association, said there was very little prison staff could do to stop Huntley attempting suicide again.
He said: "The staff at Wakefield and the governor of Wakefield know exactly what they need to do to both protect him and to keep him safe from attack from other prisoners and to keep him safe from self-harm.
"They will really press against them and you've got to keep them under review and you've got to learn from each incident and event... but I've no doubt he'll do it again."
Huntley first tried to kill himself in June 2003 after saving up 29 anti-depressant pills in a box of teabags while awaiting trial for the murder of the two schoolgirls.
An official report into the 2003 suicide attempt said Huntley presented an "ongoing significant risk of self-harm".
In September 2006 he spent a day in hospital after being found unconscious in his cell following an overdose.
That incident prompted an urgent review of how Huntley was supervised by the head of the Prison Service's Standards Audit Unit, Rob Kellett.
Huntley has also been subjected to attacks from fellow inmates.
In October last year he was reportedly held at knifepoint after being ambushed on his way back to his cell after a meal.