Phenelzine Drug Interactions: What to Avoid and What to Tell Your Doctor

Phenelzine (Nardil) has one of the most extensive and serious drug interaction profiles of any medication in psychiatry. Many combinations can cause life-threatening emergencies, including hypertensive crisis and serotonin syndrome. This guide explains what to avoid, what to watch for, and what every phenelzine patient must tell their healthcare team.

Why Phenelzine Has So Many Drug Interactions

Phenelzine permanently blocks MAO-A and MAO-B enzymes. These enzymes don't just break down neurotransmitters — they also break down tyramine, certain drugs, and a range of other compounds. By blocking both enzyme subtypes, phenelzine prevents normal drug metabolism and amplifies the effects of many substances to dangerous levels.

Contraindicated Drugs: Never Combine With Phenelzine

The following drug combinations with phenelzine are absolutely contraindicated — meaning they should never be used together, ever:

SSRIs (selective serotonin reuptake inhibitors): Fluoxetine (Prozac), sertraline (Zoloft), paroxetine (Paxil), citalopram (Celexa), escitalopram (Lexapro). Can cause serotonin syndrome — a life-threatening condition with fever, seizures, and coma. Wait 14 days after stopping phenelzine before starting any SSRI (5 weeks if starting fluoxetine). Wait 14 days after stopping an SSRI (5 weeks after fluoxetine) before starting phenelzine.

SNRIs (serotonin-norepinephrine reuptake inhibitors): Venlafaxine (Effexor), duloxetine (Cymbalta), desvenlafaxine. Same serotonin syndrome risk as SSRIs. 14-day washout required in both directions.

Other MAOIs: Tranylcypromine, isocarboxazid, selegiline. Never combine two MAOIs. At least 14 days between stopping one MAOI and starting another.

Sympathomimetics: Amphetamines, cocaine, methylphenidate (Ritalin), pseudoephedrine, ephedrine, dopamine, epinephrine, norepinephrine. Can cause acute, severe hypertensive crisis.

Meperidine (Demerol) and some opioids: Can cause severe, unpredictable reactions. Meperidine is absolutely contraindicated. Fentanyl, tramadol, and other opioids should be used only with extreme caution if at all.

Dextromethorphan (DXM): Found in many over-the-counter cough medicines (Robitussin DM, NyQuil, etc.). Can trigger severe serotonin syndrome. Read all OTC cough and cold labels carefully — avoid any containing DXM.

Buspirone (Buspar): Combination with phenelzine can cause hypertensive reactions.

L-tryptophan, L-tyrosine, L-DOPA: These amino acid precursors can dramatically increase neurotransmitter levels when MAO is blocked, risking serotonin syndrome or hypertensive crisis. Many supplements contain these.

High-Risk Drug Combinations (Use With Extreme Caution)

Tricyclic antidepressants (TCAs): Clomipramine is absolutely contraindicated. Most TCAs require extreme caution. Nortriptyline may be used in very specific situations by experienced specialists with careful monitoring.

Barbiturates: Phenelzine enhances barbiturate effects. Doses of barbiturates must be reduced significantly.

Insulin and oral hypoglycemics: Phenelzine may enhance hypoglycemic effects. Monitor blood sugar carefully.

Antihypertensives: Phenelzine can potentiate blood pressure-lowering effects, increasing risk of hypotension.

OTC Medications to Watch Out For

Over-the-counter medications can be surprisingly dangerous on phenelzine. Specifically avoid:

Any cough/cold product containing dextromethorphan (DXM) — check the label on NyQuil, DayQuil, Robitussin, Mucinex DM, Theraflu, and all cold medicines

Decongestants with pseudoephedrine or phenylephrine (found in Sudafed, many sinus products)

St. John's Wort (a serotonergic herbal supplement)

5-HTP supplements (serotonin precursor — risk of serotonin syndrome)

Anesthesia and Surgical Procedures

If you are having any surgical procedure, tell your anesthesiologist that you take phenelzine before surgery. Many general anesthetics, muscle relaxants, and perioperative medications interact with MAOIs. In elective procedures, your surgeon may recommend stopping phenelzine 14 days before surgery.

Tell Every Healthcare Provider About Phenelzine

Every healthcare provider you see — dentist, urgent care, ER, specialist — needs to know you take phenelzine. Carry a medication card and list phenelzine on all medical intake forms. Do not assume providers know the interaction risks.

For information on phenelzine's general side effects, see our post on phenelzine side effects. And if you need help finding phenelzine in stock, medfinder can call pharmacies near you and text you which ones have it.