Is there any drug interaction between topical terbinafine and azathioprine?
That is hard to say, it depends to some extent on how much terbinafine is absorbed through the skin and gets into the rest of the body. Each drug has liver toxicity potential, and the toxicity of azathioprine increases if the liver is impaired to eliminate it. Terbinafine (lamisil) (when taken orally) has an initial half life of 12 hours but over time the half life goes up to 200 to 400 hours, meaning its elimination rate and storage in tissues increase over prolonged use. If your applying this over a wide area of your body for a long time, then it would add up much more quickly. So, whatever the risks of drug interaction there are between the two drugs, prolonged use of terbinafine might increase this risk with rising levels in the body.
