Some antibiotics can worsen some neuromuscular disorders like Myasthenia Gravis (neuromuscular junction disorder) and Hypokalemic Periodic Paralysis (muscular calcium or sodium channels disorder), especially Macrolides that are sometimes used in CFS/ME patients (azythromycin, erythromycin, clarythromycin) (1).(2) These two diseases are also worsen by low potassium, and low potassium increases toxicity of these antibiotics.(3) The mechanism is not known but alteration of muscles calcium channels or neuromuscular junction has been suggested. (4) I want to point out that Tetracyclines (Minocyclin, doxycyclin), if they are not known to worsen these diseases by itself, " should not be used at the same time as neuromuscular blocking agents since these antibiotics can increase the strength of the neuromuscular blocker" (5) This means that association between Macrolides and Tetracyclines may increase toxicity over the neuromuscular function of some patients. ME/CFS patients should be aware of this, particularly if they have muscular issues (weakness, heavy legs and arms, cramps, myoclonus, etc) or/and borderline/low potassium blood levels. I wish I had known all this when I started azythro + doxy treatment and ended with severe muscle weakness and low potassium. I hope this will be useful to others that consider any long term antibiotic treatment. I won't say not to do it, but to pay attention to muscles signs. Keeping blood potassium not below 4.2 mmol/l might be protective.