Products
Metronidazole is commercially available for the external treatment of rosacea as a cream (Rosalox, Perilox), a cream tinted with iron oxides (Perilox color), and a gel (Nidazea, Rozex). In some countries, e.g. Germany, a lotion is also available. The effectiveness of the therapy was first demonstrated by Nielsen in 1983. In many countries, the first cream was launched in 1987. Metronidazole can also be used perorally in the form of tablets to treat rosacea, but significantly more adverse effects must be expected.
Structure and properties
Metronidazole (C6H9N3O3, Mr = 171.2 g/mol) is a nitrated imidazole derivative. It is a white to yellowish crystalline powder that is sensitive to light and sparingly soluble in water.
Effects
Metronidazole (ATC D06BX01) results in a significant reduction in lesions (papules, pustules, skin infiltrations) and redness compared with placebo. The effect on telangiectasias (visible blood vessels) is considered absent to mild. The exact mechanism of action is not known, but it is believed that the effect is due to the antioxidant, anti-inflammatory, and immunosuppressive properties of the active ingredient. Gel and cream seem to be equally effective. The quality of the clinical studies performed has been criticized several times, but topical metronidazole is considered effective and one of the 1st choice agents in this indication. A Cochrane review states: “The quality of studies evaluating rosacea treatments was generally poor. There is evidence that topical metronidazole and azelaic acid are effective.” (van Zuuren et al.)
Indications
For the treatment of rosacea.
Dosage
The drug should be applied thinly to the skin areas to be treated in the morning and evening, according to the professional information. The skin is washed and dried beforehand. According to clinical studies, once-daily application may also be sufficient (off-label). The usual duration of treatment is several weeks, and up to 3 weeks may elapse before the onset of action.
Contraindications
Metronidazole is contraindicated in hypersensitivity. It should be used with caution in acute or past blood disorders. For complete precautions, see the drug label.
Interactions
Metronidazole is absorbed in very small amounts when used topically. According to the SmPC, interaction with vitamin K antagonists is possible. Alcohol should not be consumed when taking metronidazole orally because an antabuse effect may occur. In the CH technical information, such an interaction is not mentioned for external use and seems unlikely because of the low absorption, but according to the US and German technical information, it cannot be completely excluded.
Adverse effects
Possible adverse effects include rare local reactions such as hypersensitivity reactions, skin irritation, skin dryness, redness, worsening of skin lesions, burning, and stinging. Systemic adverse reactions to metronidazole are not usually expected but may rarely manifest as symptoms of nausea, metallic taste, tingling, and numbness of the extremities.
