One hundred and forty pharmacists are ceasing to provide methadone to former drug addicts from this morning as part of a dispute with the HSE.
The Irish Pharmaceutical Union says pharmacies involved in the scheme are not receiving enough support to help them operate safely.
The HSE says several thousand former heroin addicts, most of them in Dublin, will be affected by the action being taken by the IPU and public order incidents may increase as a result.
Minister for Health Mary Harney, meanwhile, has criticised the pharmacists, accusing them of using vulnerable patients as part of their dispute.