In drug-induced methemoglobinemia, patients present with slate-grey to blue discoloration of the skin when levels exceed 15%. Symptoms occur in proportion to declining oxygen delivery. Headache, nausea, and fatigue occur at low levels (20% to 30%). In those with coronary artery disease, dyspnea, angina, and dysrhythmias may result. Levels above 50% can cause loss of consciousness and metabolic acidosis, and above 70% may be lethal.