 The TCAs are highly metabolised by the cytochrome P450 (CYP) hepatic enzymes.Drugs that inhibit cytochrome P450 (for example cimetidine, methylphenidate, fluoxetine, antipsychotics, and calcium channel blockers) may produce decreases in the TCAs' metabolism, leading to increases in their blood concentrations and accompanying toxicity.The major factor that distinguishes SSRI's amongst one another is the inhibition of select CYP enzymes .