Some side effects of cephalexin include breathing, tiredness, joint pain--similar to MG problems. My guess is that you may be allergic, or one of the small % of folks who have the extra side effects from this antibiotic--and that is happening, making you feel the MG is getting worse. This drug is not mentioned as a problem for MG folks as far as I can find.

We have to remember that sometimes we may have a reaction to a drug that may not be related to our MG, but the symptoms are similar and so we blame MG. Tapering prednisone gives me symptoms that are quite similar to MG itself--but it is not the MG, just the prednisone withdrawal from lowering the dose.

The side effects listed for cephalexin
Common adverse effects include gastrointestinal upset such as diarrhea, nausea, vomiting, indigestion, and abdominal pain.[6]

Others may include dizziness, agitation, headache, joint pain, and tiredness. Symptoms of an allergic reaction include rash, itching, swelling, trouble breathing, or red, blistered, swollen or peeling skin.
The "other" symptoms are in less than 1 of 100 people.

Thanks Russ! Your insight is always so helpful. While at the pharmacy picking up this drug they called my doctor to ensure that this wouldn't happen since those who are allergic to penicillin (which i am and they say 5% of people) have an adverse reaction to this medicine as well. I guess i am in that 5%!

Thanks Russ. I am supposed to take 4 of these pills before cleaning teeth. Seems like a lot. I have artificial knee and hip and I think that is why. My memory is going. :(. Your comments are helpful as always. Karin

ShanAllis, why are you taking the Cephalexin? If it was an illness, that also could've been what set off the MG exacerbation. My MG is exacerbated by both mild/moderate illnesses and most antibiotics...