Malabsorption of pyridostigmine in patients with myasthenia gravis

Four patients with myasthenia gravis were under unsatisfactory control while receiving oral pyridostigmine. In each of these patients, the serum levels of this drug were below those observed in patients with myasthenia gravis who are well controlled. The strength of each of these patients improved when the serum pyridostigmine level was increased by intravenous administration of this agent. Furthermore, the rate of disappearance of pyridostigmine from the serum following intravenous administration was the same as that for control subjects and patients under good control. This demonstrates that failure to achieve adequate serum pyridostigmine levels following oral administration is due to malabsorption rather than to increased rates of tissue uptake, degradation, or excretion of the drug.