A nationwide shortage of prescription drugs is preventing a Nova Scotia woman from getting the medication she's been taking for 20 years.

Irene MacIsaac went to her local pharmacy in Sydney to refill a prescription and was told the drug wasn't available.

She was shocked by what she heard next.

"The frustrated pharmacist said, 'We have pages and pages and pages of drugs we are not able to get right now,'" MacIsaac told CBC News.

Weeks later, MacIsaac was prescribed a replacement drug. She said she'll take it, but she does have reservations.

"Hopefully alternate drugs can fill the bill, but that's not always going to be the case," she said. "There is one drug that I'm taking as well for my thyroid, Synthroid, and you wouldn't want to be off that. You have to take that on a regular basis."

MacIsaac's story is not uncommon.

For months, Nova Scotians have been getting alternate drugs or have been leaving pharmacies with their prescriptions only partially filled.

Some anti-depressants and antibiotics, including penicillin, have been in short supply across Canada. Some hospitals have even had to scrounge for chemotherapy drugs.

Jeff Morrison, with the Canadian Pharmacists Association, said there are a number of reasons for the drug shortage, including a lack of raw materials and regulatory issues.

He also wonders if a reduction in the price of generic drugs in some provinces has led manufacturers to produce smaller quantities.

One study by his group found that 90 per cent of pharmacists were experiencing drug shortages that forced them to prescribe the second, third or even fourth best medicine.

"It prevents the pharmacists from doing their job effectively, and that's a huge frustration. Pharmacists are there to provide the best quality care they can. But when the medications aren't being supplied that hampers their ability to do that," Morrison said.

He said a national drug reporting system is in the works to monitor the supply of prescription drugs.