Vega One nutritional shakes in two flavours have been found to be contaminated with a prescription drug, Health Canada says.

Testing by Vega, previously known as Sequel Naturals, of Burnaby, B.C., showed the antibiotic chloramphenicol in its Vega One Vanilla Chai (437 g and 874 g formats) and Vega Sport Performance Chocolate products.

"Sequel Naturals Ltd. is refusing Health Canada’s request to recall these products despite the potentially serious risks posed to health," the regulator said Friday.

"Health Canada is also requesting that wholesalers, distributors and retailers voluntarily hold on to any Vega One Vanilla Chai and Vega Sport Performance Chocolate currently in their possession."

In rare cases, chloramphenicol is associated with the risk of developing aplastic anemia. The condition is a serious blood disorder and it can be fatal.

Health Canada said it is impossible to predict who will develop aplastic anemia after taking the antibiotic. The risk doesn't seem to relate to the dose taken. 

Other side-effects that can be associated with chloramphenicol include allergic reactions in people who have sensitivity to the medication, and effects on the bone marrow leading to decreased blood cells. It should not be used by pregnant or lactating women.

Health Canada says it works with the health product industry to ensure hazardous products are pulled from the marketplace but recalls are initiated by importers and manufacturers themselves.

In response to an independent report into the recall of Alysena 28 birth control pills last month, Health Canada said it will explore expanded recall powers.