No significant pharmacokinetic interactions were observed between levetiracetam or its major metabolite and concomitant medications. Pharmacologically active metabolite of diazepam, q.v. Midazolam is metabolised into an active metabolite alpha1-hydroxymidazolam. It is metabolized into an active metabolite (desmethylpimobendan) by the liver. Nortriptyline is an active metabolite of amitriptyline by demethylation in the liver. Toxicity of paracetamol is believed to be due to its quinone metabolite NAPQI. Phloretic acid is also a urinary metabolite of tyrosine in rats. It is an important biochemical metabolite in plants and microorganisms. Clarithromycin (20%-40%) and its active metabolite (10%-15%) are excreted in urine. Many neonatal screening programs include methionine as a metabolite. Trimethylglycine is a functional metabolite of choline. Usnic acid is the most commonly studied metabolite produced by lichens. Its metabolism is partially hepatic, giving rise to its main metabolite, desethylchloroquine. CYP2D6 is responsible for converting fluoxetine to its only active metabolite, norfluoxetine. The CYP2C9 enzyme is responsible for the formation of the N4-hydroxy metabolite.