Prescriptions in England should be free to everyone with a long-term medical condition, the Prescription Charges Coalition campaign group has stressed..
Some people end up needing hospital treatment because they cannot afford their medication, it said.
Conditions such as Parkinson's should be added to the list of specific illnesses that require free medicines, it stressed.
But health officials say they have made sure people with chronic illnesses have access to affordable prescriptions.
The Department of Health (DH) said: 'We have made sure that people with long-term conditions can get access to affordable prescriptions.
'That's why there are a number of prescription-charge exemptions in place and 90% of items prescribed are free on the NHS in England.'
Free prescriptions in devolved nations
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.
However, 10% of prescriptions 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.
As a result, organisations such as Parkinson's UK, National Rheumatoid Arthritis Society and Asthma UK said the list of medical exemptions is 'unfair' and 'out of date'.
An unfair system?
Neal Patel, from the Royal Pharmaceutical Society (RPS), agrees that the current system is arbitrary and unfair.
'The main burden falls on those with long-term conditions such as HIV, asthma and multiple sclerosis, who have to pay for prescriptions to stay well.
'Prescription costs place a particular squeeze on the finances of those who are in work on modest incomes, who sometimes cannot afford prescription medicines - a price that's ultimately paid in preventable hospital admissions, wasted NHS time and avoidable sick leave,' he said.
Examples of current medical exemptions:
- Diabetes mellitus
- Diabetes insipidus
- Myasthenia gravis
- Disorders such as Addison's disease
- A permanent fistula such as colostomy
- A continuing physical disability that means you cannot go out without the help of another person