Is it possible that a smoker who dies from smoking at a certain age could have lived longer had he not smoked, or is one's lifespan predetermined and death singular, though its causes may vary?

Allah has decreed all destinies through causes by which they occur, and He is the Creator of both the causes and their effects. Attributing efficacy to causes is polytheism (shirk) in monotheism (Tawhid), while disregarding causes is a deficiency in intellect. Turning away from them is a flaw in the Sacred Law (Shari'a). Mere causes do not necessitate their effects; rather, the fulfillment of conditions and the absence of impediments are necessary, such as rain and vegetation, or a child and ejaculation. Allah has made consuming poisons a cause of death, but death has multiple causes. It is not possible to definitively state whether someone who died from poison, but had not consumed it, would have died at the same appointed time or not; because hypothesizing the absence of a cause is contrary to what is decreed. If what is decreed is that one dies by this specific cause, then hypothesizing the absence of this cause is contrary to what Allah has decreed.