He's given up twice. On two entirely separate occasions he has taken the last cigarette from the packet, pulled the nicotine into his lungs until the tip reached the filter, and squashed the remains into an ashtray. Last cigarettes, cold turkey. He grew used to the sound of chewing gum in his own mouth, squelching away at the craving.
On two entirely separate occasions he has started smoking again. Once when the Doctor was dying in eighteenth-century London, and once when the Doctor's heart (he only had one at the time, like Fitz) was mashed to a pulp by a stage magician. Hospitals didn't bother him before. Now he lights up at the smell of disinfectant or the shudder of a respirator.
Or the scream of a siren.
The sound vibrates through him, travelling down his spine and snaking along his fingers. He reaches into his pocket without any conscious effort, pulling out the Woodbines and the Cricket lighter.
It's behind him, racing towards his ears. The banshee wail wraps itself around him as his fingers find the catch on the lighter and a yellow flame whooshes into existence.
The Doctor isn't in the ambulance. The Doctor is walking slightly ahead of Fitz, babbling to Anji about punk rock and safety pins.
The flame finds the cigarette as the ambulance passes Fitz. The yellow light dies in the rush of air. Fitz swears, inaudible as the siren tears at his eardrums. He flinches.
The sound is the sound of the TARDIS, louder and faster. Dematerialising and disappearing and riding in an ambulance. Cracking ribs, screaming sirens.
He clicks the lighter again as the ambulance Dopplers into the distance. He sucks in the nicotine and breathes it out, grateful.
It's probably killing him. These days (Anji's days) cigarette packets tell you about the tumours in your lungs and the wheezing and the dying. Fitz should probably care.
One day Fitz will give up smoking for good, because one day Fitz won't be travelling through time.