Sources in the maximum security Maghaberry jail said the driller killer - who along with Hazel Stewart murdered his wife, Lesley, and Stewart’s husband, Trevor Buchanan - fell ill at lunchtime.
There is speculation Howell may have been on hunger strike prior to being hospitalised, Belfast Live reports .
One source said: “He has been under huge strain in recent weeks.
“The television show may well have played a part in that but it is more likely the decision to scrap his huge pension tipped him over the edge. He took that very badly”.
His condition tonight is not known.
Howell and Stewart are behind bars for murdering their spouses and making the killings look like a tragic suicide pact.
The 1991 murders were portrayed in controversial ITV drama series, The Secret, which ended last Friday night.
Belfast Live recently revealed how Howell would not receive a penny of his half-a-million-pound pension due to the sick sex acts he carried out on drugged patients at his Co Antrim clinic.
Howell - who is currently serving a life sentence - was denied a total of £587,526.27.
Just days prior to confessing to his crimes, he Googled how long he would have to serve behind bars.
Howell reckoned if he got 20 years, he could be out by the time he was 70 and could live another 20 years. He was just one year out when told he must serve at least 21.
But his carefully laid plans were scuppered when his hefty pension was axed.
A second prison source said: "We've been told Colin Howell is in a bad way both physically and emotionally.
"It's believed he has been suffering severe stomach pains and dehydration.
"He is one of Maghaberry's model prisoners but things have been very difficult for him in the last month especially because he's been under a lot pressure since the start of The Secret TV show."
The Department of Health, Social Services and Public Safety forfeited Howell’s pension on the basis that his convictions for indecently assaulting drugged patients were “liable to lead to serious loss of confidence in the public service”.
A spokeswoman for the department told us: “The Department is unable to comment on personal data.”
In May 2011, Howell was sentenced to five and a half years for sexually assaulting five female patients.
He admitted sedating the women then abusing them in the dentist’s chair of his surgery in Ballymoney between 1998 and 2008.
Howell is due to be released no earlier than 2030 and while he may have believed he was due to receive his NHS pension at retirement age, instead he won’t receive a thing.
Details of the forfeiture emerged after Howell’s estranged wife Kyle Jorgensen - mother to five of his children - instructed her legal team to enquire whether or not she was entitled to any of the pension.
Ms Jorgensen, 50, a New York native who now lives in Florida with her children, made the enquiry in a bid to seek financial security for her family.
Following “careful consideration” by the Department, it was decided that all of Howell’s HSC Pension Scheme benefits would no longer be paid.
Prior to last week’s final episode of The Secret a source close to Howell said he had just one regret about confessing to his heinous crimes - trusting Kyle to stand by him.
He believed she would support him when he finally held his hands up to the callous killings, 18 years later.
But when Howell walked into a police station and revealed the murders, Kyle fled.