Patients are finding it more and more difficult to access medicine because doctors are issuing prescriptions for drugs which are no longer available.
In some cases people have had to return to their GP three or four times for a different prescription.
Pharmacists say the issue has become an increasing problem in the last 18 months.
Community pharmacist Peter Rice, from the Ulster Chemists' Association, said the situation was extremely troublesome.
"A huge amount of time is involved, from trying to explore all options to source the drug, then often paying an excess for it if it can be sourced," he said.
"If it cannot be sourced the pharmacist has to liaise with the prescriber to arrange for a suitable alternative, all while trying to ensure the patient does not suffer."
The problem is caused by doctors prescribing pills which they believe are available, but when the patient goes to a chemist they are told the pills are no longer in supply.
The patient has to return to their doctor to get an alternative version.
Among those who have had problems accessing drugs recently is Ukip MLA David McNarry.
He went to the doctor with a minor health issue and was given a prescription, but the medicine he was allocated was not available.
Mr McNarry said he was aware of problems in his Strangford constituency, and had been told of people having to go to their doctor three or four times before getting the drugs they needed.
"The difficulty for the doctor is that they write a prescription unaware of its availability, because why would they write a prescription for something that is not available," he added.
"It is a chain of events which frustrates every one at every point. The doctor, unaware of the situation, writes the prescription, the pharmacist then has to tell the patient to go back to their doctor because the prescription is not available.
"If it was a blood pressure tablet you needed, your blood pressure would be boiling over.
"At the end of the day, the patient at the end of the queue is frustrated and angry."
The Ulster Chemists' Association said the issue had become more critical in the past 12 to 18 months.
Mr Rice said the problems were caused by various factors.
"There can be problems with manufacturers accessing raw ingredients required to make the product, there can be delays with licensing arrangements to get the drug into the market, and there are fewer generic manufacturers, so if there is a problem with a particular manufacturing plant there are fewer options to go to for alternatives," he said.
Mr Rice said the problem was not confined to specific drugs or conditions, but was a wide-ranging issue.
"Doctors and pharmacists are doing their best to manage an increasingly difficult problem," he added.
"I went to the doctor who gave me a prescription, but when my wife went to redeem it she was told the pills I required weren't available, which confirmed to me what I had been hearing from people."
Strangford MLA David McNarry