Stop Trafficking in Fentanyl Act of 2018 This bill amends the Controlled Substances Act to modify the drug quantity thresholds that trigger a mandatory minimum prison term for a defendant who manufactures, distributes, or possesses with intent to distribute fentanyl. Specifically, the bill reduces from 400 to 20 grams the fentanyl quantity and from 100 to 5 grams the fentanyl analogue quantity that trigger a 10-year or 20-year mandatory minimum prison term for high-level first-time or repeat offenders. It also reduces from 40 to 2 grams the fentanyl quantity and from 10 to 0.5 grams the fentanyl analogue quantity that trigger a 5-year or 10-year mandatory minimum prison term for low-level first-time or repeat offenders.