South Australian police and the Ambulance Service think a dirty batch of the drug ecstasy led to a spike in violent and bizarre behaviour in Adelaide over the weekend.

Authorities said there were more assaults than usual and several incidents of people stripping naked in public.

Superintendent Anthony Fioravanti said 21 people went to the Royal Adelaide Hospital with drug overdose symptoms on Saturday night and one remains in a critical condition.

"There was an increase in aggression rather than the feeling of euphoria," he told a news conference.

"This resulted in an increase in reported fights in the city and had the users displaying psychotic, extreme and bizarre behaviour, which included hallucinations and people disrobing."

Police said a North Adelaide man, 21, was arrested for drug dealing and allegedly had 33 ecstasy tablets and $900 cash on him at the time of the arrest.

He was given bail and is to face court at a later date.