A prescription tax brought in today could stop the sickest and poorest from taking their medicine, pharmacists warned.
Medical card holders have to pay the 50c levy for every item - up to a maximum of €10 a month - administered through chemists.
Darragh O'Loughlin, president of the Irish Pharmacy Union (IPU), said pharmacists remain totally opposed to the charges but have been forced to collect them for the Government.
"This levy will cause hardship to many patients, particularly the homeless and those living in sheltered accommodation, and may even prevent certain patients from taking their medicines entirely," he said.
Mr O'Loughlin pointed out prescription charges were abolished in Northern Ireland and Wales, where they were branded a tax on illness. Scotland is also phasing out the levies.
He also hit out at health chiefs for not doing enough to make people aware that the charges were coming in.
"The Health Service Executive has not run an adequate public information campaign to ensure that medical card patients would be alerted to the imposition of this charge, something which we believe should have happened long before now," he said.
Mr O'Loughlin said certain patient groups - including the homeless, patients in sheltered accommodation and nursing homes - should be exempt from the charges.
Age Action said it was concerned the tax will prevent the sickest and poorest older people - many on multiple medications - from taking all their medicine.
"We have always believed that this is a pointless charge which has potential to damage public health for no economic gain - it is not too late to reverse it," said Age Action's Eamon Timmins.
Labour health spokeswoman Jan O'Sullivan said the charges directly target the poor and the sick.
"This may not look like a big amount to the Health Minister Mary Harney and her advisers but it is another hole in the budgets of those who have to live on 196 euro per week, including disabled, blind unemployed and widowed people," she said.
Fine Gael older citizens spokeswoman Catherine Byrne TD described it as a tax on ill health which will directly hit the elderly, the disabled, the homeless and the least well off.
Sinn Fein health spokesman Caoimhghin O Caolain described the prescription charges as an attack on the poor.