The bare necessities of Ed Zwick’s dark-tinged romantic comedy Love & Other Drugs are hard to miss. Namely, the arresting sight of bright young things Jake Gyllenhaal and Anne Hathaway rolling around naked for the entire middle third of the film, flaunting their lithe and toned physiques.

Refreshingly, sex walks hand in hand with emotion in the film, which treats the physical intimacy of the characters as an important element of their developing relationship.

There’s nothing tongue in cheek or coy about the nudity — you can forget about an expertly placed vase sparing the actors’ blushes.

There is nowhere to hide for Gyllenhaal and Hathaway and they rise to the challenge admirably, catalysing a smouldering on-screen chemistry that should send temperatures soaring in multiplexes as snow blankets much of the UK.

Set in the mid 1990s, the film centres initially on pharmaceutical sales rep Jamie Randall (Gyllenhaal), who works alongside mentor Bruce Jackson (Oliver Platt), wooing waiting room receptionists like Cindy (Judy Greer) in the hope of getting their drugs on to the shelves.

During one visit to the surgery of Dr Knight (Hank Azaria), Jamie meets 26-year-old patient Maggie Murdock (Hathaway), an artist with early onset Parkinson’s disease.

The spark of attraction leads initially to sex. “I don’t want a relationship,” Maggie tells Jamie coolly. “You don’t know that. You might have some latent humanity,” he responds cheekily.

The more time Maggie and Jamie spend together, the more aware they become that her degenerative condition could rob them of any chance of lasting happiness.

Love & Other Drugs is an entertaining though curious amalgam of three different films: a forthright romantic comedy, a disease-of-the-week tearjerker, and a colourful history lesson about the introduction of a certain little blue pill to the lucrative American drugs market.

The gear changes between these three genres are sometimes crunching.

Jamie Reidy’s non-fiction book, Hard Sell: The Evolution of a Viagra Salesman, provides a loose framework and timeline for the script, but most of the turmoil springs from the work of screenwriters Zwick, Charles Randolph and Marshall Herskovitz.

Hathaway captures the physical frailties and fear of a young woman under siege in her own body, while Gyllenhaal plies his roguish charm, which is gradually undone as Jamie teeters on the brink saying those three little words.

Both capture the giddiness of lovers in the first flushes of romance, enslaved by their fragile emotions.

The grittier side of the film is exemplified in a scene at a meeting of Parkinson’s patients when one husband tells Jamie seriously to get out of the relationship while he can.