As a musical crafted from a patchwork of stories from Dr Mel Hughes’s The Parents’ Story Project, which documents the lives of parents affected by substance abuse, Score tells tales that have been told countlessly on the street and in homes. Writer Lucy Bell and director Stephanie Kempson perform a delicate, important surgery transferring them to the theatre with their integrity intact.

From trying on homemade fairy wings for their school nativity together, to peeing into a cup when scheduled to by the authorities in the hope they’ll be deemed fit mothers, the lives of old friends Hannah (Kathleen Fitzpatrick-Milton) and Kirsty (Lara Simpson) flitter by quickly on a stage which is decorated messily with KFC wrappers and children’s toys. Hannah and Kirsty always return to each other, even if they do fight. Or scream. Or rage. Or lose the other’s dog. A reunion might happen only a moment later on stage, though it may be years in the characters’ timeline. The extreme time-hopping nature of the musical feels suited to the fast-paced, seemingly out-of-control lives it documents.

But content is truly crammed into this 70-minute script and, at times, it is a tight fit with barely room to breathe: what is told in two minutes could be explored at much more depth and power in an hour-long play, to greater understanding for the audience of the characters and the issues they face. There is so much screaming and conflict, peppered with world-shifting events (world-shifting for the girls at least, that is), that sometimes it cheapens the writing and becomes confusing. This detracts from the musical’s maturity and numbs the audience to the excruciating pain being played out on stage.

Thankfully, the songs, performed a capella by Fitzpatrick-Milton and Simpson, slow the play down and are always unfailing in stilling and silencing the audience. There is nothing quite like harmonising with someone you’ve known for years, Hannah points out. Fitzpatrick-Milton and Simpson must have known each other for decades, with Simpson’s power and Fitzpatrick-Milton’s rich low notes blending beautifully. Though the transitions to song are usually seamless, the a capella covers occasional feel messily tacked on top of a straight script – less like a musical and more like a play with music. There is undeniable talent on stage: Fitzpatrick-Milton’s subtle and sensitive acting truly hits the high notes too, balancing wonderfully with Simpson’s flair for comedy.

Hannah and Kirsty are stretched and tested by addiction, violence and either the threat or the reality of their children being taken away from them (and rightly so, they sometimes think). With the blame that often wafts around drug addicts, it is refreshing to have their stories told with such consideration, honesty and charm by Score, and it is simply a bonus that it is also a great musical – moving, funny and with uplifting, beautiful singing. Though its script can be clunky, Score does it job: its final notes leave the audience caring for every real-life parent struggling with the harrowing reality of substance abuse, risking responsibility for their own children.

Score played at the Royal Festival Hall in the Southbank Centre until 23 October. For more information, see the Southbank Centre website.