Bayer Inc. and Janssen Inc., which make the drugs, are informing consumers of the problem with the fluoroquinolone class of antibiotics.
Fluoroquinolones includes Cipro, Cipro XL, Avelox and Levaquin.
Myasthenia gravis is a chronic autoimmune neuromuscular disease, the hallmark of which is muscle weakness that increases with activity and improves with rest.
Health Canada says these antibiotics can exacerbate muscle weakness in myasthenia gravis patients, sometimes to the point where it affects their ability to breath.
Health Canada says people with the condition are urged to tell their doctors and should avoid taking fluoroquinolone drugs.