A Canada-wide shortage of the asthma drug Ventolin is forcing patients to rely on a generic version of the treatment which, some say, might not be effective.

According to Alberta Health, the drug shortage is affecting pharmacies across the country, and some generic brands of medication are also in short supply.

The provincial health authority didn't say what is causing the shortage, but it told CTV Calgary that it expects stocks to be replenished around in mid-March.

In the meantime, however, patients will have to resort to using other forms of the drug.

University of Calgary student Kate Reid says her severe asthma has caused her breathing troubles for her entire life. She said she's on several medications for asthma and she relies on an inhaler for emergency situations.

But the last time Reid tried to get her prescription for Ventolin filled, she says the pharmacy gave her a generic form of the drug, Salbutamol Sulfate, instead.

Reid tried to take the substitute medication during a recent asthma attack, but she said she didn't find the replacement drug effective.

"My breathing was so laboured I wouldn't have to tell someone to take me to the hospital at that point," she said.

Fortunately, Reid found an old inhaler that still contained a few puffs of Ventolin. She took the name-brand medication and says her symptoms subsided.

Following the episode, Reid inquired about Ventolin at two separate pharmacies, where they told her the shortage was caused by a manufacturing issue.

Pharmacist Heather Walker says Cambrian Pharmacy, where she works, ran out of Ventolin in December. She said the shortage could be linked to a recall of the drug in the U.S. at the end of last year.

Walker said the U.S. Food and Drug department ordered the recall after finding that some Ventolin inhalers didn't contain enough propellant to deliver the labelled doses .

Walker said she hasn't had any complaints from patients who have switched to the generic version of Ventolin, but added that generic drugs can affect patients differently from name-brand versions, even if the active ingredient is the same in both formulas.

"Sometimes it's just the hardness of the tablet or the filler they're using in the capsules that will change the way it's absorbed," she said.

And though the differences may seem small, Reid says the Ventolin shortage and generic substitution can have serious consequences for patients with her condition.

"It's ridiculous," she said. "You're putting … peoples' lives at risk by not having this medication."

With files from CTV Calgary