Barbara Weston (Julia Roberts), left, returns home to face her overbearing mother, Violet (Meryl Streep), and put-upon younger sister, Ivy (Julianne Nicholson), in “August: Osage County,” a film adaptation of Tracy Letts’s award-winning play. (CLAIRE FOLGER)

Meryl Streep utterly dominates “August: Osage County,” the screen adaptation of Tracy Letts’s Pulitzer Prize-winning 2007 drama of family dysfunction. In it, she plays another in a series of recent roles that tap into the actress’s uncanny ability to evoke both horror and pity at her character’s flaws. It’s a list that includes Miranda Priestly in “The Devil Wears Prada,” Sister Aloysius in “Doubt” and Margaret Thatcher in “The Iron Lady” — each of which earned Streep an Oscar nomination. (She actually won in 2012 for her portrayal of the former British prime minister.)

Yet with her performance as Violet Weston, the venom-spewing matriarch of an extended family that includes three grown daughters, the actress may have outdone herself. Violet, who is suffering from mouth cancer and addicted to pain pills, is a woman given to such toxic utterances and such a corrosive style of mothering that it’s a wonder more of her relatives haven’t developed cancers of their own, simply from long-term exposure.

One of them actually has. But in a figurative sense they all have, as this intricately plotted story gradually reveals, during a gathering of Violet’s clan for one family member’s funeral. Unsurprisingly, that death is an apparent suicide — circumstantial evidence, as the film strongly suggests, that Violet is impossible to live with.

There are moments when the film, which is set in a sweltering Oklahoma August in and around Violet’s house, seems a bit overheated for the screen. The inherent theatricality of Violet’s character, which may be well-suited to the stage, is occasionally hard to take in such intimate close-ups, despite Streep’s bravura acting. The film feels claustrophobic at times, and stagy.

It helps that the supporting cast is uniformly good. Next to Streep, Julia Roberts makes perhaps the strongest impression as Violet’s bitterest daughter, Barbara, who is struggling with both a philandering husband (Ewan McGregor) and a sullen teenager (Abigail Breslin) on the cusp of making one particularly bad decision. Making Barbara’s situation worse is her own suspicion that she herself may be to blame for her family’s misbehavior.

For my money, though, the best and most nuanced performances are those delivered by Julianne Nicholson, Benedict Cumberbatch and Chris Cooper, as the tale’s most tragic victims. Nicholson breathes life into mousy daughter Ivy, the one Weston girl who — unlike Barbara and the baby of the family, Karen (Juliette Lewis) — never escaped the homestead for saner pastures. She’s left caring for Violet, and trying to carve out a small corner of life for her own happiness and healing.

Late in the film, which was adapted by the playwright for the screen and directed by veteran television producer John Wells, there’s a bombshell that gets dropped, devastating the long-suffering Ivy, and more indirectly affecting her uncle, Charlie (Cooper), and her cousin, known as Little Charles (Cumberbatch). Like Ivy, Charlie and Little Charles have already been victims in their own right, having suffered years of humiliation at the hands of Violet’s almost equally nasty sister, Mattie Fae (Margot Martindale).

But by the time the movie’s climax comes round, the combined fate of this trio is heartbreakingly sad.

Despite some moments of caustic humor, highlighted in the film’s misleading trailer, “August: Osage County” is in no way a comedy. Neither is it simply interested in wallowing in Violet’s unrelenting ugliness. Two scenes — one in which Violet recalls her own mother’s cruelty to her, and another in which she’s shown dancing, a lonely and unlovable creature who is both a survivor and a perpetrator of abuse — reveal the not-so-subtly hidden message of Letts’s play and film.

It’s a message reflected in the several generations of women at the heart of the story, from Violet’s unseen mother to Barbara’s still salvageable adolescent daughter: Monsters aren’t born, but made.

★ ★ ½

R. At area theaters. Contains crude language, sexual references and drug abuse. 119 minutes.