Institutional violence from police militarization and drug cartel wars as a ‘Big Event’ and its influence on drug use harms and HIV risk in people who inject drugs on the U.S.–Mexico border

•Drug market enforcement policies in Mexico promote drug cartel wars, which exacerbate policing, drug use harms and HIV risk.•The intersection of law enforcement militarization tactics deployed in response to drug cartel wars and violence stemming from cartel wars is a ‘big event’ that reshapes the drug market.•A ‘big event’ such as a drug cartel war reshapes the drug use and HIV risk environment and severely disrupts harm reduction efforts.