Zyleuton and zileuton CR cause elevations in liver enzymes in 2% of patients; the two drugs are therefore contraindicated in patients with active liver disease or persistent hepatic enzyme elevations greater than three times the upper limit of normal. Hepatic function should be assessed prior to initiating either of these drugs, monthly for the first 3 months, every 2–3 months for the remainder of the first year, and periodically thereafter; zileuton also has a rather unfavorable pharmacological profile (see Zileuton#Contraindications and warnings). Given these deficiencies, other drugs targeting ALOX5 are under study.