The fic you are thinking about is Blood Magic by GatewayGirl.
Not being a coddling type Snape says if Harry can break out and get to hogwarts within a certain amount of time he can stay with Severus and never return to the Dursleys:
"I won't rescue you, Potter," he sneered.
Potter shrugged slightly. His entire bearing conveyed that he had not expected it. Snape wondered that he could ever have taken that diffident fatalism for impertinence.
"But I will offer you this," Snape heard himself say. "Get out of here -- on your own, mind -- and make it to Hogwarts, and I'll see to it that Dumbledore allows you to stay."
Literally hours after Harry leaves the house it goes down in flames:
"Headmaster, what is that blinking thing?"
He had hoped to take the unflappable wizard by surprise. He was not pleased to have produced fear, even briefly. Albus Dumbledore feared almost nothing. The second of panic that crossed his face shook Severus to his barely-healed core.
"That would be ... your son's home," Dumbledore said softly. "The wards are down."
The street they materialized on smelled of dirty smoke. Regular pulses of color lit the space around them; after a moment of disorientation, Severus realized that these pulses came from rotating lights on the tops of Muggle vehicles, some of which were quite large. Beyond the vehicles was a Muggle house with noticeable fire damage. A cloud of green sparks, still recognizably the Dark Mark, hung in the sky above it, slowly losing form to the light breeze.
Harry is a smoker:
"The ... cigarettes."
"Oh." Harry thought. "I'm not sure, actually. Not much. Probably one or two a day, last week, because I was so hungry, but much less often before that. I thought the headache might be connected, but it didn't help in the least."
While making potions with Snape later in the story he makes some potion bubbles that make people high:
Several hours later, the material in Harry's cauldron had clarified and turned an iridescent pink. Weird, Harry thought. Wonder what made it that color? He pulled out the spoon to check for consistency. The liquid cohered to itself and the spoon, and slid off in a single thin stream. It looked to Harry like nothing so much as a strong soap solution. Like a soap solution, it remained stretched across the holes in the bowl of the gold spoon. Impulsively, Harry lifted the spoon up and blew on it. Twelve tiny bubbles shimmered in the air like floating jewels. Harry reached out to catch one and it broke at his touch, as did another, beside it. He giggled.
"Look what I made!" he crowed.
"What you produced," he said, "caused a brief surge of silly cheer, followed by roughly five minutes of unconcerned benevolence, both accompanied by a heightened awareness of some physical senses, at least sight and touch. After that, it faded off into an effect reasonably like a mild Calming Draught, but of shorter duration. The catnip might explain the short duration, but it is a fairly common substitution that certainly would not cause the silliness, benevolence, or heightened sensitivity. It seems unlikely that gold, which is extremely non-reactive, would do this, either."