In many hospitals, debt collectors are being employed to pursue cancer patients for the daycare fees, which are capped at €750 in one year.
The hospitals are entitled to impose the day charge under long-established legislation, but until recently most were exempting cancer patients.
Medical card holders and people with private health insurance are covered for the €75-a-day charge -- but hospitals are free to demand it from the thousands of remaining patients, said Kathleen O'Meara from the Irish Cancer Society.
The organisation has been dealing with an increasing number of calls from worried cancer patients and their relatives.
"Hospitals in Dublin and outside of Dublin are asking for the charge. Patients who contacted our helpline are extremely worried because they cannot afford it," she said.
Around 30,000 people in Ireland are diagnosed with cancer annually and chemotherapy is necessary in most cases.
The society says many cancer patients are struggling to pay bills for basics such as travel and heat while also coping with the physical and psychological effects of their illness.
By law, a patient undergoing any form of daycare procedure, who is not covered, must pay €75 a time.
A spokeswoman for the Health Service Executive (HSE) said hospitals are under huge financial pressure and are required to collect the €75 levy.
Meanwhile, new figures show that more than 75,000 hospital appointments have been cancelled over the past three years as the pressure from cutbacks and growing patient numbers takes it toll.
The HSE says the cancellations are mainly due to closure of wards and a lack of capacity caused by emergencies.
At least 25,317 day-case and 50,433 inpatient appointments were cancelled in 2010, 2011 and the first 10 months of this year, the HSE figures show.