harm reduction 2011 study opioid substitution programs harm reduction harm reduction 'Opioid substitution therapy in resource-poor settings' 2011 study harm reduction harm reduction World Health Organisation World Health Organization opioid substitution programs Backed by the World Health Organization harm reduction study Opioid substitution therapy in resource-poor settings’ La Organización Mundial de la Salud reconoce 2011 study Opioid substitution therapy in resource-poor settings’ harm reduction 2011 study