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. you should also talk with your doctor if you have questions about specific medicines that you are taking.