Boiling water works on exactly one kind of problem: clogs made of grease, fat, and soap scum. Heat melts those substances back into a liquid so they can be flushed down the line. If your kitchen sink is draining slowly after months of cooking grease and dish soap film building up in the trap, a careful pour of boiling water is a reasonable, no-cost first attempt before you reach for any tool.
The material your pipes are made of decides whether this is smart or risky. Metal pipes, copper or galvanized steel, handle boiling water without complaint. The danger is with plastic. PVC and ABS pipe is rated for water around 140 degrees Fahrenheit, well below boiling, and repeated doses of 212-degree water can soften the pipe and, worse, loosen the glued joints. A loosened joint under a sink can turn a slow drain into an active leak.
Never pour boiling water into a porcelain toilet bowl. Porcelain is a hard, glazed ceramic, and a sudden blast of boiling water against a bowl sitting at room temperature can thermally shock it and crack it. A cracked toilet bowl isn't a repair, it's a full fixture replacement, and the leak can wreck your floor on the way out. Toilets get a plunger or a closet auger, never boiling water.
When the conditions are right, technique matters. Pour the boiling water in stages rather than all at once. Send down about a third, wait 30 seconds to a minute to let the heat work on the clog, then pour the next portion. Pour straight down the center of the drain, not against the side of a plastic basin, and you send the heat where it's needed and cut contact time on vulnerable surfaces. Two or three staged pours is a fair trial.
Keep your expectations realistic. Boiling water only handles the grease-and-soap category of clog. It does nothing for hair, mineral scale, a foreign object, or a solid obstruction deep in the line, and pouring more of it won't change that. If the water just pools on top of the clog and sits there, the blockage isn't grease, and you've learned something useful: it's time for a different approach.
If two or three staged attempts don't restore normal flow, stop pouring and switch to mechanical methods or call a professional. A plunger or a hand-cranked drain snake can physically reach a clog that heat can't touch. And if the slow drain shows up at more than one fixture, gurgles, or backs up into another drain, the problem is likely in the main line, which is firmly a plumber's job rather than another kettle of water.