Five years ago, Melissa McCarthy was a jobbing stand-up, juggling time between the US comedy circuit and acting work.

Then came Bridesmaids.

Cast as an overly aggressive singleton, who surveys one potential suitor and growls, "I'm going to climb that like a tree", McCarthy unleashed a comic whirlwind that has been blowing at gale force ever since.

She earned BAFTA and Oscar nominations as Best Supporting Actress for Bridesmaids, and won an Emmy the same year for hit sitcom Molly & Mike.

Hosting stints on Saturday Night Live garnered more Emmy nominations and back-to-back film roles alongside Jason Bateman in Identity Thief and Sandra Bullock in The Heat confirmed her Midas touch at the box office.

Now, the innately lovable star produces, co-writes and headlines this brash, oestrogen-fuelled road movie, which sees her husband, actor Ben Falcone, venture behind the camera for his first stint in the director's chair.

Alas, McCarthy's golden touch doesn't extend to scriptwriting because Tammy is a hotch-potch of half-formed characters and ideas lacking nuance and depth.

It's fitting that a comedy about a forty-something woman on the brink of emotional meltdown should itself be a shambles but, as a viewing experience, Falcone's inaugural offering is more pain than gain.

The luckless heroine is Tammy (McCarthy), a dishevelled fast food restaurant worker, whose car is wrecked by a wayward deer.

She's subsequently fired by her boss and Tammy arrives home early to discover her husband Greg (Nat Faxon) enjoying a romantic meal with a next-door neighbour (Toni Collette).

Ignoring the warnings of her mother (Allison Janney), Tammy embarks on a road trip with her profanity-spewing, hard-drinking grandmother Pearl (Susan Sarandon), who has always wanted to visit Niagara Falls.

The gung-ho ladies seek sanctuary with cousin Lenore (Kathy Bates) and her partner Susanne (Sandra Oh), and enjoy the company of cowboy Earl (Gary Cole) and his son Bobby (Mark Duplass), who takes a shine to Tammy.

"I don't think putting two messes together is going to make an unmess," she declares profoundly.

With each new misadventure, Tammy slowly realises she is mistress of her destiny.

Clumsily scripted and poorly paced, Tammy huffs and puffs with good intentions but barely raises a smile.

McCarthy works tirelessly but she's on a hiding to nothing.

Misery is heaped upon the titular protagonist to the point of absurdity, which wouldn't matter if Tammy was a fully fleshed, endearing creation, but she bellyaches and gripes, without any urge to remedy her dire situation.

As Pearl acutely observes, "Every time something bad happens, you throw a fit!"

The second half softens Tammy with the introduction of Duplass' nice guy, who evidently sees positive qualities in her that we can't and almost don't want to.