Former MP Evan Harris has taken Ecstasy for a Channel 4 show which will examine the effects of the class A drug.
He joins previously announced names, actor Keith Allen and novelist Lionel Shriver, in the controversial show.
Drugs Live: The Ecstasy Trial sees the participants given MDMA or a placebo before being placed inside a brain scanner on two separate occasions.
Dr Harris, a former Liberal Democrat MP who lost the seat for Oxford West and Abingdon in the 2010 general election, told the Radio Times that he wanted to help find a dose that might treat depression without giving the patient a high. He admitted: "In order to be allowed to take part in the trial, I needed to have taken Ecstasy at least once before without any ill effects. Without going over my 'Ecstasy history', I qualified."
He added: "It was not obvious to me when I was taking the MDMA and when I was taking the placebo. But it was clear I wasn't raving... I wasn't entering into this hoping to get a free high. I was just keen to help the study."
The study is being led by Professor David Nutt, the UK's former chief drugs adviser who was sacked after criticising government policies.
Channel 4's David Glover, who commissioned the show, defended the programme saying: "Obviously we don't want to be part of glamorising drug use. This is quite a brave and radical piece of television, which probably only Channel 4 would do. But at the same time, the programmes will be about really rather sophisticated science."
The show - which will also involve a female vicar and ex SAS man who have not yet been named - has previously been criticised by Julia Manning, chief executive of independent think-tank 2020 Health, as "reckless and pointless".
In a later statement, Dr Harris said: "I've long been an admirer of the work done by Professor David Nutt in educating the public about the effects, and potential risks, of drug taking. So I was happy to volunteer to take part in the six-month long scientific study... to look at how MDMA affects the brain."
He added: "Alongside the study, Channel 4's Drugs Live: The Ecstasy Trial programmes offer a valuable opportunity to discuss the issues surrounding ecstasy and a unique chance to give people balanced, evidence based information about this drug."