Lighting tar heroin, opium, morphine, oxy or meth on a sheet of foil, then following the smoke with a straw and then inhaling. Hence, chasing the dragon.
It can also be a metaphor for an addict trying to get the feeling of that very first high, which we all know never happens.
The OP mentioned a sweet smell, which indicates opium or black tar heroin. Although opiates, they are highly refined, so morphine and oxy will just smell like burning pills.
Meth being smoked smells like a cross between a toxic fire at a dump and the New Jersey Turnpike. Lovely, isn't it?