Q: Ciprofloxacin is not a good choice in patients with myasthenia gravis. Why?

Answer:  Ciprofloxacin is now known for more than 20 years to cause worsening of myasthenia gravis, including muscle weakness and breathing problems - and may require ventilatory support.

Other drugs which make MG worse include Beta-blockers, Lithium, Procainamide, Verapamil, Prednisone and Neuromuscular blocking agents.

Reference: http://www.accessdata.fda.gov/drugsatfda_docs/label/2011/019537s075,020780s033lbl.pdf