Pharmacology & Drug Interactions

Why pharmacology review is non-negotiable

Every serious adverse-event pattern reported for ibogaine- and MDMA-type therapies traces back to one of two preventable failures: a drug-metabolizing enzyme that concentrated a cardiotoxic compound in a patient's blood, or a serotonergic combination that was missed at intake. Established Neither failure requires exotic pharmacology to catch — it requires a complete medication list, working knowledge of CYP2D6, and the discipline to say "not yet" until a taper has been safely completed. This module gives you the mechanisms, the checklists, and the boundaries of your own authority under the Readiness Standard: you screen, you reconcile, the prescriber directs the taper — and the treating clinician alone signs the clear-to-treat.

CYP2D6 metabolism and poor-metabolizer dosing

CYP2D6 makes up under 2% of the liver's total cytochrome P450 content but handles roughly a quarter of all commonly prescribed drugs — antidepressants, antipsychotics, opioids, beta-blockers, and several of the medicines covered in this program. Established It is also the most polymorphic CYP enzyme in humans. Phenotype is usually described in four bands:

• Poor metabolizer (PM) — little to no functional enzyme; drugs cleared by CYP2D6 accumulate, and prodrugs activated by CYP2D6 (codeine, tramadol) barely convert to their active form.
• Intermediate metabolizer (IM) — reduced activity; partial accumulation.
• Normal/extensive metabolizer (NM/EM) — the population reference range.
• Ultrarapid metabolizer (UM) — extra functional gene copies; drugs clear unusually fast, and prodrugs can over-activate.

Genetic PM prevalence ranges from under 1% to about 5% of the population depending on ancestry, but that undercounts real-world exposure: pooled across intermediate, poor, and ultrarapid phenotypes together, roughly a third of patients worldwide carry a non-normal CYP2D6 status. Established Plan for an abnormal phenotype in your caseload — it is not a rare edge case.

Ibogaine is the clearest high-stakes example in this field. It is converted by CYP2D6 to its active metabolite, noribogaine. In a controlled pharmacokinetic study, healthy volunteers whose CYP2D6 was pharmacologically blocked (using the inhibitor paroxetine to simulate a poor-metabolizer state) showed roughly double the combined ibogaine-plus-noribogaine exposure of normally metabolizing subjects, with parent ibogaine detectable for far longer. Established The study's own recommendation: genotype before treatment, and at minimum halve the intended dose in confirmed poor metabolizers. Its design also makes a second point that matters just as much clinically — that "poor metabolizer" state was produced by a drug, not a gene. A patient taking a strong CYP2D6 inhibitor is functionally phenocopying a poor metabolizer for as long as that drug is on board, regardless of their own genotype.

Serotonergic interactions and serotonin syndrome

Serotonin toxicity is a spectrum, not a single fixed threshold — it runs from mild tremor and agitation to a life-threatening triad of neuromuscular hyperactivity, autonomic instability, and altered mental status, with core temperature exceeding 41°C (106°F) reported in severe cases. Established The most widely validated bedside tool is the Hunter Serotonin Toxicity Criteria, which anchors the diagnosis on clonus — spontaneous, inducible, or ocular — in a patient who has taken a serotonergic agent, distinguishing it from look-alike syndromes such as neuroleptic malignant syndrome (rigidity-predominant, slower onset, without prominent clonus). Established

The medicines in this program are not equally risky in every combination. True serotonin toxicity most reliably occurs with a serotonergic overdose, or when a serotonergic agent is combined with a monoamine oxidase inhibitor (MAOI) — that combination is the one most consistently linked to severe and fatal presentations. Combining two non-MAOI serotonergic agents (an SSRI plus MDMA, for example) still carries real risk and is still avoided before treatment, but sits on a lower rung of the same gradient. Established That distinction does not soften the bottom line: every combination below still requires prescriber-directed clearance, with no exceptions made on the strength of "it's probably fine."

• MAOIs (phenelzine, tranylcypromine, isocarboxazid, selegiline at oral/higher-dose exposure) — highest-risk category with any added serotonergic agent, including MDMA, psilocybin, and LSD.
• SSRIs and SNRIs — the everyday drivers of missed interactions; must be identified and addressed before any strongly serotonergic treatment.
• Easy-to-miss serotonergic agents at intake: tramadol, dextromethorphan (common in OTC cough/cold products), linezolid (a weak, reversible MAOI), triptans, St. John's Wort, and lithium (potentiates serotonergic toxicity without being serotonergic itself).
• Ayahuasca is worth understanding even outside this program's covered medicines: it works only because its harmala alkaloids are themselves MAOIs, protecting orally-taken DMT from first-pass breakdown. That is a deliberate pharmacologic mechanism, not a safer version of the interaction — by design, it carries the same MAOI-combination risk described above.

Per-medicine pharmacology, quick reference

• Ibogaine — hERG potassium-channel blockade prolongs the cardiac QT interval and can cause fatal arrhythmia at therapeutic doses, even without pre-existing heart disease; CYP2D6-dependent conversion to noribogaine, an active metabolite detectable for days, means cardiac risk outlasts the acute dosing window. Established
• MDMA — releases and blocks reuptake of serotonin (and norepinephrine/dopamine), driving serotonin-syndrome risk with SSRIs/SNRIs/MAOIs; independently raises core temperature and blood pressure/heart rate; partially metabolized via CYP2D6, and MDMA itself transiently inhibits its own CYP2D6-mediated clearance at higher doses. Established
• Ketamine/esketamine — NMDA-receptor antagonist metabolized mainly via CYP3A4/CYP2B6 to norketamine; interactions cluster around CYP3A4 inducers/inhibitors rather than CYP2D6. Separate risks: urothelial/bladder toxicity with repeated use and dependence potential. Esketamine (Spravato) is FDA-approved for treatment-resistant depression under a REMS program; ketamine itself is used off-label. Established
• Psilocybin — rapidly dephosphorylated to psilocin, a serotonin (5-HT2A) agonist and a monoamine oxidase substrate; MAOI co-administration can prolong and intensify exposure, and combination with other serotonergic drugs carries the class-wide serotonin-toxicity risk described above. Established
• LSD — metabolized substantially through CYP2D6 and CYP3A4; CYP2D6 status and interacting drugs may alter the intensity and duration of an already long (up to ~12-hour) session. Plausible
• Cannabis (THC/CBD) — CBD is a clinically relevant inhibitor of CYP3A4 and CYP2D6 and can raise levels of many co-administered drugs; THC/CBD are otherwise metabolized via CYP2C9/CYP2C19/CYP3A4. Established
• Kratom — mitragynine is metabolized via CYP3A4 (and, to a lesser extent, CYP2D6); opioid-receptor activity means combination with other opioids, benzodiazepines, or CNS depressants raises respiratory-depression risk, and hepatotoxicity has been reported independent of drug interactions. Established

Full medication reconciliation

Medication reconciliation for this population has to go further than a standard intake med list, because the failure modes are specific: a missed CYP2D6 inhibitor, a forgotten OTC cough syrup, or an unreported supplement can be the entire difference between a routine session and a cardiac or serotonergic emergency. Established

Washout considerations, done safely

A washout is a medical taper, not a waiting period the patient manages alone or that this program manages for them. Established It is planned, dosed, and monitored by the treating prescriber, weighing the interaction risk being avoided against the real risk of destabilizing a condition the medication was treating — stopping an SSRI/SNRI is not risk-free, and abrupt discontinuation carries its own withdrawal syndrome and relapse risk.

• General SSRIs/SNRIs: conventional practice allows roughly five half-lives to clear before starting a strongly serotonergic treatment or an MAOI — in practice, about two weeks for most agents in this class.
• Fluoxetine is the outlier: its active metabolite, norfluoxetine, has a half-life long enough that a five-week washout is the standard recommendation before starting an MAOI or comparably serotonergic treatment — a plain two-week rule under-clears this specific drug.
• MAOIs: because inhibition is often irreversible, roughly two weeks off the MAOI is needed before starting a new serotonergic agent, to allow enzyme resynthesis.
• Any CYP2D6-inhibiting drug relevant to a planned CYP2D6-metabolized treatment (ibogaine, potentially LSD) should be cleared on its own pharmacokinetic timeline, separate from the serotonergic-washout clock — these are two different mechanisms with two different clocks, and both need to run out.

Documentation, authority, and what this standard does not do

Everything in this module feeds one document: the clear-to-treat packet — screening results, full medication reconciliation, contraindication review, and washout completion, reviewed and signed by the treating clinician. Established That signature is the only thing that clears a patient. The readiness process, this course, and any screening checklist document risk; none of them substitute for clinical judgment, and none of them constitute an endorsement of any medicine's safety or efficacy. Nothing in this module should be read as a claim that ibogaine, MDMA, or any other substance covered here is proven safe or effective — several remain Schedule I controlled substances with no FDA-approved indication, and the pharmacology above exists to reduce preventable harm during an already-occurring, clinician-directed treatment, not to endorse the treatment itself.