Tyramine is physiologically metabolized by monoamine oxidases (primarily MAO-A), FMO3, PNMT, DBH, and CYP2D6.Table 5: N-containing drugs and xenobiotics oxygenated by FMO</ref> Human monoamine oxidase enzymes metabolize tyramine into 4-hydroxyphenylacetaldehyde. If monoamine metabolism is compromised by the use of monoamine oxidase inhibitors (MAOIs) and foods high in tyramine are ingested, a hypertensive crisis can result, as tyramine also can displace stored monoamines, such as dopamine, norepinephrine, and epinephrine, from pre-synaptic vesicles. Tyramine is considered a "false neurotransmitter", as it enters noradrenergic nerve terminals and displaces large amounts of norepinephrine, which enters the blood stream and causes vasoconstriction.