Pangdemonium's upcoming production The Effect considers whether love and other human emotions are only chemistry. In the tight, four-actor play, volunteers testing a new anti-depressant are increasingly attracted to one another as they receive larger doses of the drug.

Exploring the chemical basis of one's feelings has deeper meaning for two of the lead actors, Linden Furnell and Nikki Muller. They play the test subjects, observed by bickering doctors (Tan Kheng Hua and Adrian Pang) in the play, which runs at Victoria Theatre from Feb 25 to March 13.

In real life, Furnell, 27, has been on an anti-depressant for three months. Muller, 30, prefers alternative therapies for depression and is seeing a counsellor.

"If you've figured out the formula for human happiness, does it devalue the happiness?" asks Furnell in an interview at the Ubi Road offices of Pangdemonium, which is helmed by Pang and his wife Tracie.

"Chemically, there is zero difference between chemically procured happiness and happiness derived from other external sources," Furnell says.

"There's the question: 'Is it real?'" says director and producer Tracie Pang, 48. "It felt real to that person at the time."

In 2013, Pangdemonium staged Next To Normal, a musical about mental illness which received critical and popular acclaim. In it, Furnell played the boyfriend of a character whose mother has bipolar disorder. Director Tracie Pang recalls her surprise at the response to the show.

"We got personal letters saying: 'Nobody's ever talked about that before. This is my life being shown. I can show this to my family,'" she says. "It was eye-opening to see how many people were affected."

So now Furnell and Muller are sharing their experiences with depression, hoping it will help others in the same place. Muller talks about how easy it is to access therapy via Care Corner Service Centres around Singapore.

Furnell says: "I'm a pretty high- energy person but it's different behind closed doors. Euphoria is bound to exhaust you."

The anti-depressant helps put him on a more even keel.

Anti-depressants can promote the production of neurochemicals also produced when a person falls in love, which is the basis of The Effect, written by critically acclaimed British playwright Lucy Prebble.

The play was first staged in 2012 at the National Theatre in London. It was restaged there last year - the Pangs failed to get tickets but decided to perform it - and also had an amateur staging in Singapore last September via young troupe Couch Theatre.

Prebble's chemical analysis of love is the main attraction for many and was echoed in Pangdemonium's casting process. Actors were paired off in speed-dating style until the couples with the best chemistry were revealed.

In spite of the science in The Effect, love remains a mystery for both the young couple discovering a mutual attraction as well as the older doctors, who have their own history.

When it comes to love, older does not mean wiser, says Adrian Pang. His character is a "realistic romantic" embarking on a new relationship. "He's a scientist, his job is to look at things empirically, but as a person, he loves the idea of love."

Tan says the age difference between the two pairs of characters - as well as the actors, she is 53 and Adrian Pang, 50 - adds poignancy to her character's uncertainty about love. "Our not knowing comes from a different place, from experience," she says.

Asked whether the premise of The Effect is scary, Muller says: "That's the understatement of the century." She does not mean the idea that love can come from a prescription drug but the parallel theme about the ethics of drug trials. She brings up recent real-life news reports of patients killed or left brain-dead by pharmaceutical tests.

Given its sobering themes, The Effect appears to be the antithesis of Valentine's Day soppiness. Yet it is also very funny with "wonderfully romantic lines", according to Adrian Pang. He thinks it would make a great date night show.

"It would give people a lot to talk about," he says. "We don't want to over-analyse what love is and yet this makes you see there's an A+B formula."

Tan has a slightly different take. "There are lots of unanswered questions at the end of the play, but one thing which is quite conclusive is that the heart never fails to surprise people. You can try to put human beings into an Excel file or a graph, but in the end, they always surprise you," she says.