Noribogaine: The Metabolite That Outlasts the Drug

Anyone reading about ibogaine for long enough meets a second name. It matters more than it looks, because most of the time a person spends with this drug in their body, the drug is no longer ibogaine.

What the conversion is, and who does it

Ibogaine loses a methyl group in the liver and becomes noribogaine. The enzyme responsible was identified in human liver tissue in 1998, and the finding was unusually clean: a single enzyme, CYP2D6, accounted for more than 95 per cent of the clearance.

The same study included tissue from a donor who was a poor metaboliser. That sample had only the slow, low-affinity route. The genetic variation was visible in the chemistry before anyone had given the drug to a patient.

The plant does the same reaction backwards

Calling noribogaine a metabolite is correct about people and incomplete about the molecule, because the compound exists before anyone swallows anything.

In 2018 a group at the John Innes Centre sequenced the iboga transcriptome and identified the last two steps of ibogaine biosynthesis in the plant. They named the enzymes ibogamine 10-hydroxylase and noribogaine 10-O-methyltransferase, and confirmed both by expressing them in yeast and in E. coli. The pathway they describe runs:

ibogamine → noribogaine → ibogaine

The plant builds ibogamine first, hydroxylates it at position 10 to give noribogaine, then methylates that oxygen to finish ibogaine. Noribogaine is the last intermediate before the end product.

Now put that beside the liver. CYP2D6 takes ibogaine and removes the same methyl group from the same oxygen. The final step of the plant’s synthesis and the first step of human metabolism are the same bond, run in opposite directions. The plant spends its last enzymatic act making ibogaine out of noribogaine, and the body’s first act is to undo it.

One limit on how far to take this. Being an intermediate on a pathway is not the same as accumulating in the tissue, and no published analysis of root bark reports free noribogaine as a measurable constituent. The compound in the plant is transient. What you would buy is still ibogaine, and what circulates in you for the next three days is still a metabolite. The pathway explains why the two molecules are one methyl group apart, which is not an accident of pharmacology but the plant’s own last step.

The other alkaloids of iboga covers ibogamine, which this pathway makes the parent of everything else here.

How much apart

Two human studies put numbers on it.

In twenty-one healthy volunteers, half were pre-treated for six days with a drug that blocks CYP2D6 before receiving 20 mg of ibogaine. Enzyme activity correlated strongly with exposure to ibogaine, and blocking it roughly doubled exposure to the active compounds. The authors’ recommendation was explicit: genotype people before treatment, and at least halve the intended dose in poor metabolisers.

That study used a chemical stand-in for the genotype and a dose one twenty-fifth of a treatment dose. The second study did neither.

Fourteen patients with opioid use disorder were given a real 10 mg/kg treatment dose in a Dutch hospital and genotyped. Clearance of ibogaine rose by about 30 litres per hour for each point of CYP2D6 activity score, from a baseline near zero at score zero. That is not a modest gradient. It is the difference between clearing the drug and barely clearing it at all.

The half-life is the whole point

Those noribogaine half-lives come from two studies in which it was given directly, to healthy volunteers and to opioid-dependent patients.

The parent drug is gone within a day. The metabolite is still there two or three days later. Its peak concentration is lower than ibogaine’s, roughly a third, but because it persists so much longer the total exposure over time is comparable.

This is why the acute experience ending is not the same as the drug leaving, and it is the pharmacological basis for the observation that people are discharged from clinics while still carrying an active compound.

Noribogaine and the heart

The cardiac question is where the metabolite stops being a footnote.

Ibogaine and noribogaine were tested against the hERG potassium channel, the channel whose blockade prolongs the QT interval and causes torsade de pointes, in the same set of experiments.

Two things follow.

The metabolite is the slightly more potent blocker. Not dramatically, but the direction is the opposite of what people assume when they hear the word “metabolite”. The authors of that study connect noribogaine’s long half-life to clinical observations of QT prolongation persisting for days after ingestion, and that connection is the most economical explanation available.

Plant-extracted and semi-synthetic ibogaine performed the same. The difference between those two numbers is smaller than the spread within either. Anyone claiming that naturally sourced ibogaine is cardiologically different is not describing this measurement. Where ibogaine actually comes from covers the sourcing question on its own terms.

Given on its own, it did not work

Noribogaine has been given directly to people, which spares us having to reason from the parent drug.

Twenty-seven opioid-dependent patients received 60, 120 or 180 mg of noribogaine, or placebo, in a randomised double-blind ascending-dose safety study. The result has two halves and both matter.

The QT signal was dose-dependent and linear with concentration, at about 0.17 milliseconds per nanogram per millilitre, giving mean prolongations of roughly 16, 28 and 42 milliseconds across the three dose levels.

On withdrawal, the paper reports a trend toward lower scores that did not reach statistical significance.

The awkward finding for the metabolite hypothesis

A common argument runs that noribogaine carries the lasting benefit, since it is the compound still present when people report feeling different days later.

One measurement makes that harder. Ibogaine’s highest-affinity target in the whole screened set is the sigma-2 receptor, which it binds at about 201 nanomolar. Noribogaine binds the same site at about 5,226 nanomolar, a loss of roughly twenty-six-fold.

Whatever the metabolite is doing, it is not doing that.

And the one human study to test concentration against outcome found the relationships pointing the wrong way for a simple story. Ibogaine concentrations correlated with QT prolongation and, strongly, with the loss of coordination. Noribogaine concentrations correlated with neither, which is why the authors conclude both effects are driven mainly by the parent compound. And neither compound correlated with the severity of opioid withdrawal.

The compound that tracked the harms was the parent. Nothing tracked the benefit.

What this means in practice

Three consequences follow from the pharmacology, and none of them is controversial in the literature.

1. Milligrams per kilogram is not a dose. Two people of the same weight given the same amount can differ several-fold in exposure, and the variable that decides it is rarely measured. Both research groups above recommend genotyping.
2. The monitoring window is longer than the experience. The compound associated with persistent QT prolongation peaks late and clears slowly. Ibogaine and the heart covers what that means for how long someone should be watched.
3. Interacting drugs matter more than usual. Anything that inhibits CYP2D6 pushes a person toward the poor-metaboliser end, and a great many common psychiatric medicines do exactly that.

This site publishes no dosing guidance and never will. The point of this page is narrower: the number on a clinic’s protocol describes the tablet, not the person.