Terbinafine (brand name Lamisil) interacts with a surprisingly long list of medications because it strongly inhibits one of your liver’s key drug-processing enzymes. This means dozens of common prescriptions, from antidepressants to heart medications, can build up to potentially dangerous levels in your body while you’re taking it. Beyond drug interactions, certain health conditions and substances also make terbinafine risky or off-limits.
Why Terbinafine Causes So Many Interactions
Your liver uses a specific enzyme called CYP2D6 to break down a large number of medications. Terbinafine is a potent inhibitor of this enzyme, meaning it slows your liver’s ability to clear those drugs from your system. The result: blood levels of the affected medication rise, sometimes dramatically. In one study, terbinafine increased levels of a CYP2D6-processed substance by 115% after just four days of use.
What makes this especially tricky is that terbinafine has a long elimination half-life. It lingers in your body and can continue affecting how you process other drugs for weeks, even months, after you stop taking it. Any medication that relies on CYP2D6 for metabolism is a potential concern.
Antidepressants
Many common antidepressants are processed by the same liver enzyme that terbinafine blocks. This includes several SSRIs (like fluoxetine and paroxetine), SNRIs (like venlafaxine), and older tricyclic antidepressants (like amitriptyline and nortriptyline). When terbinafine slows the breakdown of these medications, their blood levels climb, intensifying both therapeutic effects and side effects.
In one published case, a 66-year-old woman on venlafaxine and mirtazapine for depression developed psychomotor agitation and symptoms of mania two months after starting terbinafine for a toenail fungal infection. Her antidepressant levels had risen high enough to destabilize her mood. If you take any antidepressant, your prescriber may need to lower the dose or monitor you more closely during terbinafine treatment.
Heart and Blood Pressure Medications
Several beta blockers used for high blood pressure, heart rhythm problems, and anxiety are metabolized through CYP2D6. Propranolol and metoprolol are two of the most commonly prescribed examples. Terbinafine can raise their blood levels enough to cause excessive drops in heart rate or blood pressure. This interaction is rated as moderate, meaning it doesn’t automatically rule out combined use but does require dose adjustments and closer monitoring.
Certain antiarrhythmic drugs used to control irregular heartbeats also rely on CYP2D6. If you’re on any heart medication, make sure the prescriber handling your terbinafine knows about it, because the effects can persist for weeks after finishing the antifungal course.
Antipsychotics and Other Psychiatric Medications
Aripiprazole, a widely used antipsychotic, showed concentration increases of 18% to 59% in patients also taking terbinafine. That’s enough to trigger side effects like restlessness, tremor, insomnia, and nausea. Other antipsychotics processed by CYP2D6, such as risperidone and haloperidol, carry similar risks. Dose reductions of 20% to 60% may be necessary when these medications overlap with terbinafine.
Cough Suppressants Containing Dextromethorphan
This one catches people off guard because dextromethorphan is in dozens of over-the-counter cold and cough products (Robitussin DM, NyQuil, Delsym, and many store brands). Terbinafine increased the ratio of dextromethorphan to its breakdown product by 97-fold after 14 days. That’s not a subtle change. Elevated dextromethorphan levels can cause dizziness, drowsiness, and in extreme cases, serotonin-like symptoms. Check the active ingredients on any cold medicine before taking it with terbinafine.
Caffeine
Terbinafine reduces caffeine clearance by about 19%. For most people, this is mild enough to go unnoticed. But if you’re a heavy coffee drinker or sensitive to caffeine, you might notice more jitteriness, insomnia, or a racing heart than usual. You don’t need to quit caffeine entirely, but cutting back slightly during treatment is a reasonable precaution if you notice these effects.
Alcohol
There’s no specific pharmacological interaction between terbinafine and alcohol in the way there is with the medications above. However, terbinafine carries a serious liver toxicity warning. Cases of liver failure, some requiring transplants and some fatal, have occurred in people taking terbinafine, including people with no prior liver disease. Alcohol adds its own strain on the liver, so combining the two increases the overall burden. Most practitioners recommend minimizing or avoiding alcohol during a terbinafine course, which typically lasts 6 to 12 weeks for toenail infections.
Pre-Existing Liver or Kidney Disease
The FDA label is direct: terbinafine tablets are not recommended for patients with chronic or active liver disease. The severity of liver-related side effects is worse in people who already have compromised liver function. Baseline liver testing is recommended before starting treatment, with a follow-up test about one month in.
Kidney disease also matters. In people with significant kidney impairment, terbinafine clearance drops by roughly 50%, meaning the drug accumulates to higher levels than intended. If you have kidney problems, your doctor needs to weigh whether the benefit of treating a fungal infection justifies the risk, and may adjust the dose accordingly.
Pregnancy and Breastfeeding
Terbinafine is generally avoided during pregnancy. Since onychomycosis (the toenail or fingernail fungal infection it’s most commonly prescribed for) is not a medical emergency, treatment is typically postponed until after delivery.
During breastfeeding, the picture is more nuanced. Oral terbinafine does pass into breast milk, though at relatively low levels. An exclusively breastfed infant would receive about 3.8% of the mother’s weight-adjusted dose. Some medical sources still recommend avoiding oral terbinafine while nursing, particularly with younger infants who are more vulnerable to liver effects. Topical terbinafine is considered lower risk since only about 1% absorbs through the skin, though you should avoid applying it near the nipple area.
Cyclosporine
If you take cyclosporine, an immunosuppressant commonly used after organ transplants or for autoimmune conditions, terbinafine increases its clearance by about 15%. This is the opposite of most interactions on this list: instead of making the other drug build up, terbinafine causes cyclosporine levels to drop, potentially reducing its effectiveness. For someone relying on cyclosporine to prevent organ rejection, even a 15% decrease can be clinically meaningful.
How To Protect Yourself
The most important step is giving your prescriber a complete list of everything you take, including over-the-counter cold medicines, supplements, and psychiatric medications. Because terbinafine’s effects on drug metabolism can persist for weeks after you finish the course, interactions don’t end the day you take your last pill. Any dose adjustments to other medications may need to stay in place for a period after terbinafine treatment ends.
Watch for new or worsening symptoms during treatment, particularly unusual fatigue, dark urine, or upper abdominal pain (which can signal liver trouble), as well as increased side effects from any other medication you take. Liver function testing at baseline and about a month into treatment helps catch problems before they become serious.