Methemoglobin: NO is absorbed into the blood where its binds to the ion of the heme protein, subsequently producing nitrosyl-hemoglobin, which is oxidized to methemoglobin with the release of nitrates  . Methemoglobin levels should be measured frequently and kept <%. With standard iNO doses, methemoglobinemia is unusual. In the Neonatal Inhaled Nitric Oxide Study Group (NiNOS) trial, the peak level of methemoglobin was %±%  . Premature infants are at a higher risk of methemoglobin toxicity due to reduced levels of methemoglobin reductase, but at iNO doses of <20 ppm, methemoglobin levels were not elevated   .
The most commonly reported side effects were: oral thrush , nausea , headache , and pain in the pharynx or larynx . More rarely reported side effects (occurring in <1% of patients during the clinical trial) include: tachycardia , palpitations , dry mouth , allergic reaction ( bronchospasm , dermatitis , hives ), pharyngitis , muscle spasms , tremor , dizziness , insomnia , nervousness , and hypertension . Patients experiencing an allergic reaction or increase in difficulty breathing while using this medication should immediately discontinue its use and contact their physician.