Stephen McMahon of the Irish Patients Association has said many people will not be able to afford a €50 charge that may be applied to medical cards in the up-coming Budget.
Health Minister Dr James O'Reilly has warned the charge, as well as the closure of hospital beds and nursing homes, is under consideration.
However, Mr McMahon said the levy on medical cards would put them beyond the reach of many people.
"They would (then) defer going to their GP if they don't have their medical card (and) become far sicker because they haven't been having their regular check-ups," he said.
"It could play into collapsing our hospital system as people arrive with very serious illnesses because they haven’t had their regular visits to their GP."
Pharmacists are also warning against any moves to bring in a charge for medical cards or hike up prescription levies.
The Irish Pharmacy Union (IPU) said upping the dispensary fee would put extra pressure on those who are already facing real financial hardship - and such a move could lead to people simply not taking the medicines prescribed for them.
The IPU wants vulnerable patients to be exempt from the levy if it is given the green light.