If you ever wondered what you'd be like if you weren't shy, polite, tolerant, modest, sexually repressed, logical and constrained by modern standards of hygiene, this may be the book for you. Charlotte Roche's heroine, Helen, is a wistful feminist creation, a walking, talking, bleeding, masturbating, haemorrhoid-bedecked apologist for anal sex and home-made tampons. She's not without a touch of Munchausen's, too, trying to use a self-induced hospital emergency to reunite her long-estranged parents.

This nut sees no connection between her delight in bacteria (she likes to rub her vulva all over public toilet seats, mopping up the stray pubic hairs and excretions of strangers) and the anal blister and concomitant infection that now require surgery of the bleakest kind. Most patients would subside into misery and humiliation afterwards, desperately awaiting release - either from "the ass ward" or from life itself. But Helen, despite a fear of never having a working sphincter again, embarks on an amorous pursuit of one of the nurses, and a campaign to spread her blood, germs and pee throughout the hospital.

In quieter moments she tends her avocado garden, which she forced her mother to transport to the hospital. A row of avocado pits stand sentinel to our heroine's antics and, apart from being used occasionally as dildos, strike a quiet, restrained note in contrast to Helen's feverish mixture of horniness, confusion, indignation and bloody-minded good cheer.

As with Chuck Palahniuk, there's a consistent - and somewhat formulaic - endeavour here to gross you out. Helen is keen to inform us, repeatedly, that every squeezable, drainable, detachable substance produced by the body (hers, her lovers', or yours) can be and should be eaten - except hair, which she shaves off weekly, and ear wax, for which she shows unexpected disdain. There's no mention of belly button fluff either - but blackheads, snot, puke, pus, scabs, tears, smegma, eyelid crumbs, vaginal discharges, menstrual blood and other gunk are all acceptable fodder, especially when dried to a crust under the fingernails. "I'm my own garbage disposal. Bodily secretion recycler," she tells us proudly. The passage in which she rips open her own wound to prolong her stay in hospital is even more challenging for the weak-stomached reader.

Helen is 18 and still at school. Her previous hospital stays include a bout of appendicitis she faked in order to postpone a French exam, and a sterilisation her mother knows nothing about. She's pretty angry at her mother, not just about the divorce, but about other crimes too, from mild maternal interference to suicide attempts. Helen chooses to see her much duller father as utterly blameless - apart from the way he used to administer sun cream, leaving white question marks on her sunburnt back every summer.

The presence of the parents provokes corny psychology lessons on dysfunctional families, and Helen's originality and ingenuity seem less remarkable when attributed to family trauma. Why doesn't Roche bravely proclaim her heroine's outlook NORMAL? Let Helen be promiscuous, impetuous and insubordinate because she wants to be, not because there's anything wrong with her or her childhood. There's a failure of nerve here.

The novel monotonously never exits the hospital setting. And Helen has a women's magazine info streak that sometimes shakes one's faith in her kookiness. Do we really need such handy tips as a lecture on the superiority of dabbing pussy juice behind the ear instead of perfume? She's good on hospital gowns, though: "Why does this piece of clothing even exist?" Or the obtuse ineptitude of doctors: "Is he crazy? He's the one who did this to me. I didn't mess around with his ass."

Given the outrageousness of her subject matter, though, it's a letdown that Roche's humour fails to build to a Rabelaisian pitch. This is not a beautiful or perfect book, but an enterprising one, and its cumulative effect is admirable: through Helen's all-out absorption in her physical self, her encyclopaedic demonstration of its properties, we glimpse how deeply attached we are to our bodies. We all have a relationship with zits, shits, nits and pregnancy kits: a private world of untold fun. Our bodies mean a lot to us - even the asshole, about which far too little has been written. Every writer needs to claim a bit of territory, and assholes are there for the grabbing. Boldly, Roche takes them for her own.

• Lucy Ellmann's latest novel is Doctors and Nurses (Bloomsbury).