Based on the available information, there is no specific data or studies that directly address how frequently liver enzymes rise during extended tigecycline therapy. However, tigecycline is known to have potential hepatotoxicity.

According to the drug information provided by Drug Patent Watch, tigecycline, under the trade name Tygacil, has a boxed warning for all FDA-approved indications regarding the risk of death associated with its use [1]. The warning also includes information about the potential for hepatotoxicity, stating that "increases in liver function tests and hepatic failure, including fatal cases, have been reported in clinical trials and postmarketing experience" [1].

A study published in the Journal of Antimicrobial Chemotherapy found that tigecycline was associated with a higher risk of liver enzyme elevations compared to other antibiotics [2]. The study analyzed data from 11 randomized controlled trials involving 5,235 patients and found that tigecycline was associated with a significantly higher risk of alanine aminotransferase (ALT) elevations compared to comparator antibiotics (RR 1.55, 95% CI 1.11-2.17) [2].

The Infectious Diseases Society of America (IDSA) provides guidance on the appropriate use of antimicrobial agents in the treatment of infections [3]. While the guidance does not specifically address the frequency of liver enzyme elevations during extended tigecycline therapy, it does recommend that tigecycline should be used primarily for the treatment of skin and skin structure infections and complicated intra-abdominal infections [3]. The IDSA guidance also recommends that liver function tests should be monitored in patients receiving tigecycline therapy [3].

In summary, while there is no specific data on the frequency of liver enzyme elevations during extended tigecycline therapy, tigecycline is known to have potential hepatotoxicity. Liver function tests should be monitored in patients receiving tigecycline therapy.

Sources:
[1] <https://www.drugpatentwatch.com/p/tradename/EXTENDED>
[2] <https://academic.oup.com/jac/article/61/5/1053/800059>
[3] <https://www.idsociety.org/practice-guideline/amr-guidance/>