Are there medications a patient with mg should not take?

Yes. Aminoglycosides, magnesium sulfate, penicillamine, interferon-alpha, statins. Use with caution: beta blockers, fluoroquinolones, neuromuscular blockers, high dose steroids.
Talk with your doc. This is a link to an article that lists all the possible medications that may exacerbate myasthenia gravis. http://www.ispub.com/journal/the-internet-journal-of-neurology/volume-10-number-2/drugs-which-may-exacerbate-or-induce-myasthenia-gravis-a-clinician-s-guide.html you should also talk with your doctor if you have questions about specific medicines that you are taking.