Hmm why do you smoke so much? How come you can't over dose on me? whats is the meaning of this sentence

You can't overdose on cigarettes however hey are bad for your heart and lungs

It's not likely you'll overdose on nicotine just from smoking cigarettes. Your body absorbs the nicotine in a cigarette around 1 milligram when you smoke it. Overdosing from nicotine gum or a patch is rare but it's possible if you don't follow the instructions carefully.