Three young men were taken to hospital after it was believed they took a potentially lethal drug bought on the internet.
The three students took mescaline, best known as a drug used by some native Americans in Mexico as part of religious ceremonies.
Most users chew the button-shaped "seeds" found in the peyote cactus to produce the hallucinogenic effects, which can last up to 18 hours.
The trio - aged 21, 20 and 18, who live in St Christopher's Close, Chichester, West Sussex - became unwell on Saturday.
The 21-year-old was arrested on suspicion of possession and supply of a Class A drug, possession of a Class B drug, and allowing premises to be used for supplying a Class A drug.
He was bailed until July 12, while the two others will be questioned by officers after they have made a better recovery.
Detective Constable Ceri Spratt, of Sussex Police, said: "These drugs are illegal and potentially lethal.
"We would urge people not to buy drugs from the internet, even so-called legal highs, and if anyone has information about the supply of these drugs to contact us immediately."
Mescaline has been used for thousands of years, but its effect on users can include changes in time perception and altered thinking.
Vomiting, headaches and feelings of anxiety are also typical symptoms suffered by those who take the drug.