An Essex police officer who kicked an arrested man in the face did not use excessive force, a misconduct tribunal has ruled.
PC John Senior, 38, said he acted "instinctively" to defend himself at Chelmsford Police Station in 2016.
But the panel ruled the officer breached police standards of authority, respect and courtesy by using abusive language towards the detainee.
He was ordered to take "management advice" to improve his self-control.
During the five-day Essex Police misconduct hearing in Chelmsford, PC Senior said he would have been "neglecting his duty" if he failed to intervene after the arrested man tried to "throttle himself" in a cell on 1 November.
Chairwoman of the panel, Alexandra Robson, said they had considered the evidence "with great care" but found "the use of force was necessary in the circumstances".
She added that PC Senior had shown himself to be "calm, balanced and professional".
CCTV footage showed a struggle between the two men, and the use of bad language by both.
The officer, who apologised for his choice of words, said the detainee was "one of the most unpredictable people" he had ever met, but denied having used excessive force to restrain him.
The tribunal did not hear any evidence from the detainee.
Tim Johnstone, a civilian trainer employed by Essex Police, told the hearing PC Senior's behaviour had been "appropriate" and "reasonable" and he "should not have done anything differently".
Craig Rush, defending PC Senior, described him as an "experienced officer" who was "regarded well".
He added: "He is far from perfect, but his behaviour that day was reasonable, proportionate and lawful."
Stephen Morley, counsel for Essex Police, said: "We don't accept that force was inevitable as there were so many other obvious options. This is not how police officers are supposed to deal with people."