Blood pressure should be monitored closely to confirm that the desired antihypertensive effect is achieved. Methyldopa can potentiate the hypoglycemic effects of tolbutamide by impairing the drug's metabolism. Iron salts have been reported to dramatically reduce the oral absorption of methyldopa. Methyldopa can cause psychosis if administered concomitantly with levodopa. Additive hypotensive effects also could occur.
Patients receiving lithium and methyldopa concomitantly can develop lithium toxicity.
Interestingly, lithium levels may appear to be in the therapeutic range while signs of lithium toxicity are evident. Therefore, plasma lithium concentrations are not an accurate indicator of lithium toxicity in patients receiving concurrent methyldopa therapy. The most frequently reported adverse effect of methyldopa therapy is drowsiness. This usually occurs within the first 48-72 hours of administration and may subside with continued therapy. Larger doses produce sedation, decreased mental acuity, lapses of memory, and impaired ability to concentrate.