Based on the provided sources, tigecycline is an antibiotic used to treat various bacterial infections. The FDA-approved prescribing information for tigecycline is available in the third source [3].

The recommended dosing interval for tigecycline is as follows:

- For patients with normal renal function, the recommended dosage of tigecycline is 50 mg every 12 hours.
- For patients with severe renal impairment (creatinine clearance less than 30 mL/min), no dosage adjustment is needed.
- For patients with hepatic impairment, no dosage adjustment is needed for mild or moderate hepatic impairment. However, for patients with severe hepatic impairment, a reduced dosage of 25 mg every 12 hours is recommended.

It is important to note that tigecycline has a long half-life, and steady-state concentrations are achieved after the third dose [1]. The clinical study discussed in the second source [2] evaluates the efficacy and safety of tigecycline in patients with complicated intra-abdominal infections and provides additional information on the drug's usage, but it does not specifically address dosing intervals.

In summary, the appropriate dosing intervals for tigecycline are:

- 50 mg every 12 hours for patients with normal renal function
- No adjustment needed for patients with severe renal impairment
- 25 mg every 12 hours for patients with severe hepatic impairment

Sources:
[1] <https://www.drugpatentwatch.com/p/tradename/TIGECYCLINE>
[2] <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3675903/>
[3] <https://www.accessdata.fda.gov/drugsatfda_docs/nda/2013/021821Orig1s031.pdf>