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.

[0] https://www.dea.gov/resources/facts-about-fentanyl

[1] https://www.drugs.com/dosage/fentanyl.html