With intravenous administration, the onset of anticonvulsant action is fast and lasts about 30 minutes. Following intramuscular administration the onset of action is about one hour and lasts for three to four hours. Magnesium is excreted solely by the kidneys at a rate proportional to the plasma concentration and glomerular filtration. Magnesium sulfate is associated with several minor side effects; serious side effects are uncommon, occurring at elevated magnesium serum concentrations > 7.0 mEQ/L.  Serious toxicity can be counteracted with calcium gluconate.