The crash happened at about 08:00 near Whitburn in West Lothian.
The drivers sustained minor injuries, Police Scotland said.
Traffic was backed up between junctions four and five eastbound, causing long delays. The route was fully reopened after more than two hours.
The Scottish Fire and Rescue Service cut one of the drivers free from a Peugeot van.
The drivers are being treated at Wishaw General Hospital.

The Troop, by Canadian author Craig Davidson, was announced as the winner of the Â£2,000 prize on Wednesday night.
The award was set up to celebrate the life and work of horror writer James Herbert who died in March 2013.
Herbert's daughter Kerry, who was one of the judges, described the winning book as the "darkest of tales".
"My father would have chuckled in his chair; his fans will love it. And you'll never go camping again," she said.
"The Troop is a brilliant and terrifying classic that I am proud to champion as the first winner of the James Herbert Award for Horror Writing - it's now one of my favourite books."
The book was written by Davidson under the pseudonym of Nick Cutter.  It tells the story of scoutmaster Tim Riggs and his troop of boy scouts who encounter an "emaciated stranger" on a camping trip in the Canadian wilderness.
The chair of judges, Tom Hunter, said: "The Troop is a perfect first winner, and the judges loved its tense plotting, detailed characterisation and above all the driving sense of fear that compels you to keep turning every horror-soaked page until the end."
Born in Toronto, Ontario, Davidson's other works include his first short story collection, Rust and Bone, which was adapted into a film by French director Jacques Audiard starring actress Marion Cotillard as a killer whale trainer.
Herbert was the author of 23 novels, including The Rats, The Fog, The Dark, The Survivor, The Magic Cottage, Haunted and Fluke.

Since 2010 there have been more than 1,800 reports of attacks involving corrosive fluids in the capital.
It is thought acid is becoming a preferred weapon for gangs due to its ease to obtain and lack of trail to the perpetrator.
Last year it was used in 454 crimes, compared to 261 in 2015, according to Met Police figures obtained by the BBC.
Almost a third of these attacks were carried out in the borough of Newham in east London.
Men are twice as likely to be victims of acid attacks in London than women, it was found.
The vast majority of cases never reach trial, according to the figures.
Since 2014 about 74% of investigations have been wound down due to problems with identifying perpetrators, or victims being unwilling to press charges.
According to figures obtained through a Freedom of Information request to the Metropolitan Police Service "corrosive fluids" were used in murders, robberies and rapes.
Wayne Ingold was attacked in 2014 in a case of mistaken identity after opening the door to a teenager.
"The pain on my face was indescribable. My face was melting and turning yellow," he told BBC London.
"One of the police officers was physically sick when he saw my face.
"Internally I'm scarred as well. For ages I was scared of reprisals. I had to move because of it and lost my home.
"It turned out the boy who did it was 15, and a convicted dealer of cocaine."
Since the start of 2017 there have been 49 crimes where "corrosive fluids" were thrown with "intent to cause grievous bodily harm" or to "cause an explosion".
Dr Simon Harding, Criminologist and expert on gangs at Middlesex University, said there was anecdotal evidence gangs are switching from carrying knives to acid.
"It's not prohibitive to carry bleach, you can buy it online or any DIY stores, it's difficult to prove any illegal motive in carrying it and you can hide it or disguise it in a drink bottle," he said.
"A knife attack is attempted murder, but if you're caught in an acid attack it would be GBH.
"There's no specific offence regarding acid attacks" he added.
Four drivers are being treated in hospital after being injured in a crash involving three lorries and a van on the M8.
[NEXT_CONCEPT]
A novel about a scout camping weekend that takes a scary twist has won the first James Herbert award for horror writing.
[NEXT_CONCEPT]
The number of acid attacks in London has risen sharply in the past four years, according to new figures.