In opera, everyone from Wagner to Gilbert & Sullivan has resorted to drink to drown their sorrows. As ENO prepares to dispense a new `Elixir

of Love' at the Coliseum tonight,

Mike Ashman checks out the contents

on the label.

WS Gilbert called it "the lozenge", and annoyed Arthur Sullivan more each time he wanted to use the device. But Gilbert knew enough about operas grand to poke fun at their libretti with unerring success. He parodied both what he admired and what he saw succeeded. He was aware of what in this century Alfred Hitchcock would call "the McGuffin" - the hidden motor that drives the plot. Gilbert knew that the magic transforming liquids so beloved of opera plots (especially those originating in Paris) were essential targets for his accessible satire.

Two seminal 19th-century "lozenge" or potion operas are Donizetti's l'Elisir d'Amore (1832) and Wagner's Tristan und Isolde (1865). The action in the Donizetti is in fact launched by a jokey recital of the Tristan story. In Scribe's The Filtre, the original on which Felice Romani based his libretto for Donizetti's Italian comedy, Terezine (Adina) tells her villagers: "Queen Iseult had turned her back on love. But Sir Tristram was pining away for her - without so much as a word of complaint. Then, or so the old chronicle tells us, a famous warlock had him drink a magic philtre. They called it the love-drink - a potion whose secret virtue would inspire eternal love. Is its recipe really lost for ever?"

Donizetti's opera plays on a less exalted social level than the sexual politics of Arthurian royal families. But its hero Nemorino ("little nobody") is experiencing similar problems to Tristram with its heroine Adina. He takes his troubles to the visiting quack doctor Dulcamara ("sweet bitter"), demanding the love potion of Queen Iseult. Thinking on his feet, Dulcamara gives Nemorino an appropriately relabelled bottle of bordeaux.

Comically, he is convinced that drinking it himself will make his Adina- Iseult fall for him. She does - but only after the prospect of Nemorino's becoming unattainable (the village girls discover he's inherited a million) provokes jealousy and some rethinking. Comically, too, both Dulcamara (seeing naive, tongue-tied Nemorino surrounded by women) and Nemorino himself (now having to fight the girls off) believe for a short time that this wine really is a successful love potion. Then Adina tells Dulcamara that she has a more potent potion: her own sexuality.

Elisir is a classic potion opera - but its potion is a hoax. The comedy suggests a psychological truth about these magic liquids that holds even in "serious" dramas: they are not supernatural chemicals effecting psychological transformations of Jekyll and Hyde literalness. Rather are they symbols, and agents, of a release from inhibition and responsibility. No idea or sentiment, however deeply hidden or blocked, will be uncovered that was not present before. There is no magic.

Adina is fond of Nemorino, but is too close to his received image in a narrow community to realise it. Isolde and Tristan (in Wagner's version) have already fallen in a kind of love - even though he has killed her betrothed, and is escorting her into a loveless marriage to an older man. Realising that their situation has become hopelessly entangled, Isolde summons Tristan and offers him what she believes - and he quite evidently realises - to be an elixir of death. They prepare for a double suicide, but are soon in each other's arms declaring passionate love. Isolde's maid has switched elixirs. Comedy again, of a kind.

As the composer's grandson, Wieland Wagner, said, when preparing his two famously pared-down Bayreuth productions of the opera, its potion could be "pure cold water". Or not exist at all.

In 19th-century opera the potion was often a road sign recognised by its audiences: Warning! Character now departing on important mental journey. (Or, with the advantage of historical hindsight, Warning! Depth psychology ahead.) It is no surprise to find that parodies abounded.

Gilbert found l'Elisir d'Amore as irresistible to lampoon as his contemporary audiences found it in the theatre. First came a burlesque, Dulcamara or the Little Duck and the Great Quack, which, Beggar's Opera style, used two of Donizetti's tunes with new words. Then came his first full-length collaboration with Sullivan, The Sorcerer, in which John Wellington Wells, "a dealer in magical spells", plays Dulcamara in an archetypal English village. His potion (administered in tea) works, but pairs off the wrong couples, necessitating Wells's death to achieve a happy ending.

There is more than a passing similarity - English small town mores, lovelorn vicars, potions at tea parties - between G & S's Sorcerer (1877) and Benjamin Britten's Albert Herring (1947). Britten's comic opera is the sharpest of "lozenge" parodies, because he and his librettist Eric Crozier made their potion (rum-laced lemonade) the watershed moment of grocer's son Albert's rite of passage from mother-dominated shop assistant to liberated, alcohol-imbibing, (ie normal) young man. And the music Britten provided to accompany butcher's boy Sid and baker's girl Nancy's spiking of Albert's drink, Albert's downing of it, and all subsequent references thereto was wholly derived or quoted from ... Richard Wagner's Tristan und Isolde.

ENO's `The Elixir of Love' opens tonight at the London Coliseum, WC2 (booking, 0171-632 8300).