Rachel McAdams started hallucinating when she smoked marijuana.

'The Vow' actress heard opera music in her head for five hours straight when she took the illicit drug in a bid to beat her insomnia -- on the advice of her greengrocer.

Speaking on 'Jimmy Kimmel Live', she said: "He said, 'I've got something for you.' So he takes me to the back and he pulls out this little bottle with no label and it's full of this dark green substance.

"And it's marijuana. I was like, 'Oh no, no no, I want to sleep. I don't want to get high.'

"I take the tiniest little teaspoon, brush my teeth, by the time I'm getting into bed I'm like, 'My brain, what's happening?'

"I wasn't thinking normal thoughts, I was hallucinating. I was lying there cursing him.

"And then I started to hear opera for five hours. This guy singing opera; I was going crazy."

The 34-year-old star -- who split from actor Michael Sheen in February -- claims she was so furious she went back to speak to her local shop owner to question his methods.

She recalled: "He said, 'Oh, it didn't put you to sleep?' And then I really looked at him and his eyes were like this big (gestured with hands). I was like, 'You're a stoner.'"