I can tell the difference between who I am and a side effect. The Effect is a clinical romance. Two young volunteers, Tristan and Connie, agree to take part in a clinical drug trial. Succumbing to the gravitational pull of attraction and love, however, Tristan and Connie manage to throw the trial off-course, much to the frustration of the clinicians involved. This funny, moving and perhaps surprisingly human play explores questions of sanity, neurology and the limits of medicine, alongside ideas of fate, loyalty and the inevitability of physical attraction. Following on from the critical and commercial success of Enron, The Effect offers a vibrant theatrical exploration into the human brain via the heart. It received its world premiere at the National Theatre's Cottesloe Theatre in November 2012, starring Billie Piper and Jonjo O'Neill. It is published here in the Modern Classics series alongside an introduction by Miriam Gillinson.