Sarah Kenny was walking her Jack Russell cross Max in a park near her home in St Helens, Merseyside, on Saturday when he darted off to a nearby bush.
However, the 45-year-old realised something was wrong once they got home, with Max unable to stand, shaking and vomiting.
Veterinary nurse Sarah said: "It's a bush he always goes into so I let him off the lead to have a sniff around, he's always chasing after rabbits so when he took a while to come out it didn't raise my suspicion.
"He looked fine even when we came back from the walk, I took him home and he was fast asleep - we'd been out for at least three hours so I thought he must be shattered.
"But I went for a nap myself, and a few hours later my partner came and woke me up and told me something wasn't right with Max. He looked really nervous and shaky, he couldn't stand up without falling back over."
Sarah only realised exactly what had happened after getting a whiff of Max's sick.
She said: "He looked off his face, it did seem like he was on drugs but I was dismissing it in my head thinking 'that's not possible'.
"As soon as we started getting him ready to go to the vets he started throwing up.
"I could tell that it was chocolate which immediately raised alarm but then I smelt this really distinctive smell and I knew it was cannabis.
"I hadn't seen what he'd eaten in the bush before but when he was sick we knew straight away it must have been a lot of cake."
Vets confirmed Max had consumed a psychoactive substance and he was taken to Rutland House Veterinary Hospital in St Helens, where he was given medication to make him throw up any remains of the cake, before being placed on a drip and kept overnight under the close watch of experts.
Fortunately, Max was released the next day and made a full recovery. However, Sarah said the story could have been very different had they not reacted quickly.
She said: "There's a good chance that he would have gone into organ failure and died if we hadn't reacted as quickly as we did.
"It was awful to watch him go through, no pet owner wants to see their pet in distress but I kept telling myself he's a little fighter and he would pull through.
"It could have been someone else's dog who might not have been able to spot the signs as well or worse yet it could have been a child. There were children in the park that day and I've just got this vision in my head of a child messing around in the bushes and seeing that and thinking it was chocolate. That could have been horrific."