"Heroin is particularly toxic because of high lipid solubility, which allows it to cross the blood–brain barrier within seconds and achieve high brain levels.10
"Naloxone is also lipid soluble and enters the brain rapidly. Reversal of respiratory depression is evident 3–4 minutes after IV and 5–6 minutes after subcutaneous administration.11"
Etherington, Jeremy; Christenson, James; Innes, Grant; Grafstein, Eric; Pennington, Sarah; Spinelli, John J.; Gao, Min; Lahiffe, Brian; Wanger, Karen; Fernandes, Christopher, "Is early discharge safe after naloxone reversal of presumed opioid overdose?" Canadian Journal of Emergency Medicine, Ottawa, ON: Canadian Association of Emergency Physicians, July 2000.