Laughing in the face of M1 roadworks, we drove up to Sheffield for the third time this year for yet another Crucible-based theatre weekend. And what could be a more enjoyable and sociable way to start than by meeting up with Lady Lichfield and the young Duchess of Dudley at Wagamama for a yummy lunch of warm chilli chicken salad followed by white chocolate and ginger cheesecake. Add some Sauvignon Blanc into the mix et voilà! Instant delight.

All four of us headed off to the versatile little Crucible Studio, one of the best small acting spaces anywhere, which, rather like the Menier, lends its own personality to any production lucky enough to take place there. The Studio’s current offering is The Effect by Lucy Prebble, which won the Critics Circle award for Best New Play in 2013, when it was originally produced by the National Theatre. This is the first time I’ve seen anything by Ms Prebble – we missed ENRON, much to our dismay. But I can verify she is a writer of great wit and imagination, and that The Effect is a fascinating, thought-provoking play that intrigues, amuses and horrifies in equal measure.

I’ve never been involved in a drug trial. I don’t think I know anyone who has been involved in a drug trial. And, having seen The Effect, I’m not sure I would ever want to. The scene is a science lab, where Connie and Tristan, amongst unseen others, have volunteered for a trial of a new drug, which will require their undivided presence and compliance for four long weeks. No mobile phones, no outside contact, and oppressed by near-constant supervision. Once Dr Lorna James, who’s in charge of the trial, has satisfied herself that the volunteers are indeed suitable for the task ahead, the experiment commences. Small dosages at first, followed by regularly rising dosages of the drug on trial appear to create side effects that the doctor and the Pharmaceutical company were not expecting; and Connie and Tristan fall in love. But is the trial all it seems? Is the doctor as in control as she seems? Is the pharmaceutical company as open about the trial as they seem? And is the future rosy for the two young lovers?

The play is so beautifully and subtly written that you can interpret many of its events in different ways. For example, there’s the question of the placebo. If one of the clinical study participants is taking placebo rather than the drug, then it can’t be the drug that’s causing the side effect – can it? But maybe no one’s on placebo. Maybe it’s not only the drug that’s on trial here. And what happens if someone accidentally overdoses? Supposing one of the candidates hasn’t been fully truthful about their medical history? Supposing the pharmaceutical company and/or the doctor in charge have their own private agenda? How scientific can any trial be when you’re dealing with people, because people have their own emotions, foibles, secrets; and nothing can ever be 100% watertight. Can it? You’ll go on asking these questions for hours.

Daniel Evans’ direction suggests the audience are minor participants in the trial too. The stark white chairs on the stage are the same as the stark white chairs on which the audience sit. The computer readings are displayed on large screens in all four corners of the auditorium so no matter where you sit you can see them. The fifteen minute interval is counted down on a screen both inside and outside the auditorium, daring you to be late back after your half-time Pinot; nobody was, as we didn’t want to face short shrift from Dr James. All in all, you get a great sense of everyone participating in the same experiment; it’s a real shared experience.

The cast of four give outstanding performances, fully inhabiting the intricately drawn characters that Lucy Prebble has created. Ophelia Lovibond is simply stunning as Connie. Careful to conceal aspects of her current relationship and resentful of questions that she considers are too personal, she appears nevertheless willing to play the clinical trial game to the best of her ability. But you never quite know what her attitude to any event, any question, or any situation might be. You can read in her eyes as she processes new items of information, that she is working out what her reaction is going to be. My guess is that in every performance she is probably understanding anew each time what her character is going through; and you, the audience, are accompanying her on that rather savage journey. Emotional, anxious, uncomfortable; Ms Lovibond takes Connie through a gamut of reactions, before finally becoming a changed person; one with a purpose in life that she had previously lacked.

She is matched with an equally brilliant performance by Henry Pettigrew as Tristan. Where Connie is initially reserved and careful, Mr Pettigrew presents Tristan as an instantly self-confident, flirtatious charmer; a natural rule-breaker (not the kind of person you’d really want on a clinical trial!), a pusher of boundaries, a loveable rogue, with more than a side-dish of lock up your daughters about him. Mr Pettigrew interprets him as a really credible, adult version of a naughty schoolboy, encouraging other classmates to skip lessons and sneak off into an out-of-bounds area where they will get up to no good.Together the two have a wonderful chemistry, and you’d swear they were either in love in real life or really, really good actors. As the play progresses and the balance of power between the two characters changes, so that Connie is more in control and Tristan’s fortunes have declined, the love still continues, albeit more in an “in sickness or in health” vein. Nevertheless, I note with amusement the first appearance of a stagey “masturbation under the bedclothes” scene since Miss Julie Walters did it to the late Richard Beckinsale in Mike Stott’s Funny Peculiar back in 1976; although if I remember rightly, her provision of erotic stimulation wasn’t limited just to her hand. You can’t beat a good “providing sex to a patient” scene for shock comic purposes.

Connie and Tristan are not the only twosome to have their problems in this play. There’s obviously been some history between Dr Lorna James and Toby of the Pharma. It’s never made totally clear quite what went on between them, but as a result Dr Lorna has something of a tenuous grasp on sanity; and, like Tristan, but in her own way, she too falls foul of the Pharma client. In a slightly heavy use of symbolism, Toby continues on, wrecking lives one way or another, where you might otherwise traditionally expect the drug company to look after people’s wellbeing. Priyanga Burford gives a mesmerising performance as Lorna, the doctor with a steely eye for the accuracy of the trial but who begins to fray around the edges as her ability to control comes into question.And Stuart Bunce is splendidly disconnected as Toby, ostensibly reasonable and professional, but hurting too; and with just the right lack of empathy not to notice the trail of destruction in his wake.

A fascinating play, with first class performances in a stunning production. What’s not to like? It’s running until Saturday 18th July – unmissable.