Purpose of Review <p>As the overdose crisis evolves, it is important to monitor fentanyl consumption patterns. This review provides an overview of recent findings regarding illegally manufactured fentanyl (IMF) availability, use, and associated harms in the US.</p> Recent Findings <p>Availability of IMF has increased, especially in pill form, and the increasing adulteration of IMF with veterinary tranquilizers such as xylazine complicates overdose response. Prevalence in the general population based on self-reported IMF use is rare, and likely underestimated. Transitions from injection to smoking have been documented in recent years, particularly in the western US. Fentanyl-stimulant polysubstance use has also been observed increasingly among IMF-related overdose deaths.</p> Summary <p>Shifts in routes of administration, availability of counterfeit pills containing fentanyl, and common adulterants add complexity to the landscape of IMF use and related harms. Additional data is needed for monitoring changes in consumption patterns to inform prevention and harm reduction efforts. </p>