A number of drugs can affect myasthenia gravis,often by increasing muscular weakness. This is, strictly speaking, a drug-disease interaction, but such effects may be expected to oppose actions of drugs used to treat myasthenia gravis. A number of drugs (e.g. chlorpromazine,methocarbamol, and propafenone) are clearly contraindicated in patients with myasthenia, and, as this is not strictly a drug interaction, they
See Beta blockers + Anticholinesterases,p. 834.
Persisting myasthenic symptoms,including muscular weakness, attributed to prior chloroquine use. Development of myasthenic symptoms in 3 patients,one who took chloroquine in overdose.
Aggravation of myasthenic symptoms in a patient taking pyridostigmine,and unmasking of myasthenia in one patient.
8,9
20,21
Carmignani M,Scoppetta C, Ranelletti OF, Tonali P. Adverse interaction between acetazolamide and anticholinesterase drugs at normal and myasthenic neuromuscular junction level. Int J Clin Pharmacol Ther Toxicol (1984) 22,140–4.
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are not dealt with here. A number of case reports (see table 1 below, (above)) describe worsening or unmasking of myasthenia gravis with a range of different drugs. The evidence for many of these interactions is very sparse indeed,and in some instances they are simply rare and isolated cases. It would therefore be wrong to exaggerate their importance, but it would nevertheless be prudent to be alert for any evidence of worsening myasthenia if any of drugs listed are added to established treatment.
| Table 1 Case reports of drugs aggravating or unmasking myasthenia gravis | ||
|---|---|---|
| Drug | Effect seen | Refs |
| Acetazolamide 500mg intravenously | Aggravation of muscular weakness in patients with myasthenia gravis taking unnamed anticholinesterases. | 1 |
| Ampicillin up to 1.5 g daily | Aggravation of myasthenic symptoms in 2 patients taking pyridostigmine. | 2 |
| Aspirin | Mild aggravation of myasthenic symptoms in a patient taking neostigmine. | 3 |
| Beta blockers | See Beta blockers + Anticholinesterases, p. 834. | |
| Chloroquine | Persisting myasthenic symptoms, including muscular weakness, attributed to prior chloroquine use. Development of myasthenic symptoms in 3 patients, one who took chloroquine in overdose. | 4-7 |
| Ciprofloxacin | Aggravation of myasthenic symptoms in a patient taking pyridostigmine, and unmasking of myasthenia in one patient. | 8, 9 |
| Dipyridamole* 75mg three times daily | Aggravation of myasthenic symptoms in a patient taking distigmine. | 10 |
| Erythromycin 500mg intravenously | Precipitation of a myasthenic crisis in an undiagnosed 15-year-old girl. | 11 |
| Imipenem/cilastatin 500mg four times daily | Aggravation of myasthenic symptoms in a patient taking pyridostigmine. | 12 |
| Ketoprofen 50mg daily | Aggravation of myasthenic symptoms in a patient taking neostigmine. | 3 |
| Lithium carbonate 600mg daily | Unmasking of myasthenia in one patient. | 13 |
| Norfloxacin* | Aggravation of myasthenic symptoms in a patient taking pyridostigmine. | 14 |
| Penicillamine | Aggravation of myasthenic symptoms in numerous patients taking anticholinesterases. Amitriptyline and imipramine also implicated in 2 cases. | 15-18 |
| Phenytoin 100mg three times daily | Aggravation of myasthenic symptoms in an untreated patient. | 19 |
| Procainamide* 250mg | Serious aggravation of myasthenic symptoms in a patient taking pyridostigmine. Two other less severe cases also reported. | 20, 21 |
| Quinidine* up to 970mg daily | Mild aggravation of myasthenic symptoms in one patient taking pyridostigmine and another taking neostigmine. Development of myasthenic symptoms in 2 undiagnosed patients. | 21-23 |