A letter from consultants at the Beatson Oncology Centre in Glasgow criticises the "postcode lottery" for treatments which has continued despite a shake-up supposed to improve access to rare-disease medicines.
The letter states: "We are concerned that despite the suggestions from Scottish Government that the system of application requesting use of these medicines is now less restrictive and access to these drugs is more open, our local experience is that the system remains inflexible, opaque and unfit for purpose.
"It continues to give the impression that a process exists to allow patients with serious health issues access to these drugs, when in fact this is plainly not true."
The letter to MSPs on Holyrood's Health and Sport Committee suggests specialist drugs are more readily available elsewhere in Scotland.
It added: "The existence of postcode prescribing within Scotland adds to the injustice experienced by our patients."
The rules on specialist drugs for patients with rare conditions are undergoing change in preparation for the introduction of an all-new system in April.
The new system is intended to replace the controversial individual patient treatment request (IPTR) process.
That allows consultants to apply on a case-by-case basis for medicines that are not routinely prescribed on the NHS.
Health Secretary Alex Neil, who announced the overhaul in October, vowed no patient would be disadvantaged during the transition period.
Mr Neil said: "I don't see any reason whatsoever why we can't have a system in Glasgow, as we have in the rest of the country, where the very flexible approach is taken.
"If a patient would benefit from a particular cancer drug … if the clinical view is that the patient would benefit from that particular drug, then the patient should get that drug."
NHS Greater Glasgow and Clyde said its interim policy "is very much in line with the spirit of the revised arrangements"