SINGAPORE - Over three years, a 22-year-old man posed as an ex-schoolmate of young boys aged 12 to 14 and persuaded them to consume between 10 to 20 laxative pills as part of a fictitious school experiment.

When the boys suffered from severe vomiting and diarrhoea, Sean Chew Jun Yang would ask for photos and videos of their vomit and faeces, which he used to fulfil his sexual fantasies.

He later admitted that the photos and videos made him feel "excited".

After he was arrested in 2012, he reoffended twice while out on bail, using the same modus operandi. There were 13 victims in total.

On Friday (Nov 13), Chew was sentenced to three years' jail after admitting to eight counts of causing hurt by means of poison. Another 10 charges were taken into consideration.

Chew is out on a $20,000 bail and intends to appeal his sentence. Now 26, he is currently in his final year studying for an engineering diploma.

He targeted his first group of victims, then aged between 12 and 14, by first befriending them on Facebook in early May 2012.

He adopted several monikers on two Facebook accounts and pretended to be their primary school friend. Later, he convinced the boys to participate in a fictitious school project, which involved consuming at least 10 pills of laxative drug Dulcolax at a time and informing him of its effects.

Taking deliberate attempts to conceal his identity, Chew would leave the Dulcolax pills at pre-arranged spots in public areas such as in staircase landings, for the victims to retrieve later.

He then instructed the boys to consume 10 to 20 pills at a go, about five to 10 times the recommended dosage. They all developed severe bouts of stomachache, diarrhoea and vomiting after that.

To ensure they had taken the pills, Chew would conduct Skype video calls with some victims while covering his webcam with masking tape to hide his face.

Each time, he would ask for photos and videos of the victims' faeces and vomit.

Chew had sought to obtain live photos and videos from these secondary school boys to derive sexual gratification, as he was not content with the available material online.

He was arrested after after some of the boys told their parents about his experiment. One of the victims had also alerted his form teacher.

While out on bail last year, Chew committed the same offence on a 14-year-old boy and reoffended again in June this year.

Asking for a four to five years' jail term, Deputy Public Prosecutor Andre Chong told the court that Chew had "systematically and repeatedly preyed on young boys for his own sexual gratification".

Noting that four of the victims had consumed a total of 20 pills in a day while one was made to consume 30 pills in two days, he said Chew showed a "complete lack of regard for their safety".

The recommended dosage of Dulcolax for children and adults is one or two tablets at night.

In mitigation, defence lawyer Sunil Sudheesan said his client has been suffering from a fetishistic disorder, which was left undiagnosed and untreated until last year.

District Judge Liew Thiam Leng turned down the defence's call for probation, noting the severity of Chew's offence, his reoffending behaviour and the need for the court to protect the public.

The judge said it was "necessary to have him (Chew) detained in an institution where rehabilitation can still be carried out".