Both things can be true, and are in the case of fentanyl. The lethal dose is small in absolute terms, perhaps as low as 2mg[0]. The therapeutic window is large in that doses around 50mcg[1] are useful for pain relief in adult patients. That's 1/40 the potentially lethal dose.
Are you thinking of Oramorph? I know a bunch of older ladies who use it because they have nerve pain, it's derivative of poppies/opium so it's morphine rather than fentanyl.
Because the whole point of fentanyl is the massive therapeutic window and difficulty of overdosing.