What is terbinafine?
Terbinafine is an allylamine medicine used to treat fungal infections. It is especially effective against dermatophytes (tinea infections).
The original brand is Lamisil®. In New Zealand (2016) it is available as 250 mg tablets (Dr Reddy's Terbinafine, fully funded by PHARMAC) and Terbinafine-DRLA. The tablets require a doctor's prescription. Topical options include creams, gel, solution and spray from various manufacturers, and are available at a pharmacy without a prescription.
How does terbinafine control fungal infections?
Terbinafine inhibits a fungal enzyme, squalene epoxidase, and stops the cells making ergosterol, the main component of the cell wall.
What is the usual dose regime for terbinafine?
- Topical terbinafine is applied to the affected area once or twice daily for one to four weeks.
- The oral dose of terbinafine for adults is 250 mg daily.
For children, the tablets can be hidden in food – the tablets taste unpleasant:
- Weight 10–20 kg, 62.5 mg per day
- Weight 20–40 kg, 125 mg per day
- Weight > 40 kg, 250 mg per day
Sometimes, if the fungal infection does not clear, the dose in children may need to be increased.
- Tinea corporis, tinea cruris, tinea pedis, tinea manuum: 1 to 4 weeks
- Tinea unguium: for 6-8 weeks (fingernails) or 3-4 months (toenails)
Treatment can be repeated if necessary.
What is the metabolism of terbinafine?
Terbinafine is absorbed well when taken orally, with or without food. It is bound to proteins such as albumin in the circulating blood and becomes concentrated in fat cells and within skin and nails. It is slowly eliminated in faeces and urine mostly after conversion by the liver into inactive compounds. It is a moderate CYP2D6 inhibitor. Doses may need to be reduced in the presence of kidney disease.
Skin concentrations may be up to 75-fold higher than those in the blood. It may persist in the skin for up to 8 weeks after the drug has been discontinued and in the nails for up to a year.
Side effects of terbinafine
Terbinafine appears to be a relatively safe drug. Side effects, usually minor, arise occasionally. Serious side effects occur rarely. Routine blood test monitoring is not considered necessary .
Adverse reactions to terbinafine may include:
- Nausea, a feeling of fullness
- Diarrhoea, abdominal pain
- A headache
- Painful muscles and joints
- Taste disturbance, loss of appetite (uncommon, <1%).
- Liver disease (rare, <0.01%).
- Reduced neutrophil white blood cell count (very rare, <0.01%).
- Allergic skin rash including urticaria (common, <10%) and Stevens-Johnson and toxic epidermal necrolysis (very rare, but potentially dangerous, <0.01%).
Oral terbinafine should not be taken in pregnancy or when breastfeeding. Topical terbinafine is regarded as safe to apply when breastfeeding provided the infant does not come into direct contact with the medication.
Drug interactions with terbinafine
Terbinafine does not generally alter the concentration of other medications.
Rifampicin results in a slight decrease in the concentration of terbinafine. Cimetidine may slightly increase the concentration of terbinafine. Terbinafine may increase the concentration of aripiprazole, brexpiprazole, darifenacin, eliglustat, vortioxetine, tricyclics and several other medications. It may reduce the analgesic effectiveness of codeine and similar drugs. Terbinafine is not thought to interact with the oral contraceptive pill.