Precursor to metabolite: Doxepin low-dose (insomnia) to Nordoxepin

Doxepin low-dose (insomnia) is converted to Nordoxepin by CYP2C19.

The pair

Doxepin low-dose (insomnia) — Very-low-dose doxepin for sleep-maintenance insomnia; H1 blockade dominates.
Nordoxepin — Doxepin metabolite, more noradrenergic and longer-lived than the parent.

Anything that inhibits or induces CYP2C19 shifts the balance between Doxepin low-dose (insomnia) and Nordoxepin.