On Monday night on ITV, David Tennant appeared as the title character Dennis Nilsen in the new drama Des, which documented the killer's arrest and conviction.
The three-part series started with Dennis being arrested for murder after killing at least 12 men between 1978 and 1983 in London.
Viewers watched as Dennis went into detail about the murders, openly discussing the victims and listing them while showing no remorse.
They flocked to Twitter just minutes into the show to share how sick they were feeling by David's convincing portrayal of the serial killer.
One said: "It truly is a credit to David Tennant and how good an actor he is that his portrayal of Dennis Nilsen is actually making me feel sick."
Another tweeted: "OMG I feel really sick, I might actually have nightmares tonight. Great line up too."
A third wrote: "15 minutes into Des and I feel sick to my core."
Someone else commented: "Just shows what an incredible actor David Tennant is. Not one ounce of graphic scenes but his words and acting just made me shiver and feel sick."
While a fifth added: "15 minutes in. Give all the awards to David Tennant. I'd forgotten for a second it was him. So convincing. He makes me feel sick and full of fear. So emotionless. And that right there is why David Tennant is the best actor of his generation and out there."
Nilsen confessed to killing 15 people when he was first arrested but later changed his story to 12.
Eight of his victims have been identified, with the first being shown during Monday night's episode.
He was 14-year-old Stephen Dean Holmes, who reportedly met Nilsen at the Cricklewood Arms pub before the vile killer took him to his home.
The other identified victims are Kenneth Ockendon, William Sutherland, Martyn Duffey, Malcolm Barlow, John Howlett, Stephen Sinclair and Graham Allen.
Twisted Nilsen died in prison in 2018 when he was 72, though members of the victims' families are still alive now.
Des continues Tuesday and Wednesday at 9pm on ITV