The rise of illicit fentanyls, stimulants and the fourth wave of the opioid overdose crisis.

PURPOSE OF REVIEW: This review provides an update on recently published literature on the rise of illicit fentanyls, risks for overdose, combinations with other substances, e.g. stimulants, consequences, and treatment.

RECENT FINDINGS: Overdose due to illicit synthetic opioids (e.g. fentanyl and fentanyl analogs) continues to rise in the US both preceding and during the COVID-19 pandemic. Fentanyl-related overdose is rising in new geographic areas e.g. the western USA. Stimulant-related overdose is also increasing nationwide driven by methamphetamine and cocaine. Polysubstance use, e.g. the use of a stimulant along with an opioid is driving stimulant-related overdose. Other medical consequences of injection drug use are rising including HIV and hepatitis C infections. Medication approaches to treating opioid use disorder remain the standard of care and there are new promising pharmacological approaches to treating methamphetamine use disorder.

SUMMARY: A 'fourth wave' of high mortality involving methamphetamine and cocaine use has been gathering force in the USA. Availability and use of illicit fentanyls are still the major drivers of overdose deaths and the current rise in stimulant-related deaths appears entwined with the ongoing opioid epidemic.