Prescriptions in England should be free to everyone with a long-term medical condition, the Prescription Charges Coalition campaign group says.
Some people end up needing hospital treatment because they cannot afford their medication, it says.
It wants conditions such as Parkinson's to be added to the list of specific illnesses that require free medicines.
But health officials say they have made sure people with chronic illnesses have access to affordable prescriptions.
Zoe Oakley, a bus driver and mother of two from Poole, is one of thousands of people who would like free medication.
She has polycystic kidney disease and is currently waiting for a transplant, but says the costs of her blood-pressure medication to protect her kidneys can take its toll.
She says: "There were two times when I couldn't afford them and wasn't going to refill my prescription until I got paid.
"The morning after skipping my medication, I woke up with what can only be described as feeling like the worst hangover ever.
"I had an absolutely pounding headache. Later in the day, my blood pressure spiked so high that I had to be taken to hospital by ambulance.
"The first time, I had a lumbar puncture and an MRI scan to make sure there was no bleeding in my brain.
"You cannot tell me that that was less expensive for the NHS than covering the cost of my prescription.
"And I'm currently on the transplant list for a new kidney. If I get it, I'll have to pay for my own anti-rejection medication. That doesn't seem right."
Wales, Northern Ireland and Scotland have scrapped prescription charges for everyone in recent years.
In England, many are dispensed free of charge, mostly to the over-60s, the under 16s, pregnant women and those on low incomes.
But 10% are paid for - and mostly by people on modest wages with long-term conditions not included on the current exemption list, according to the coalition group.