See the DrugPatentWatch profile for bosentan

Based on the information provided, there is no specific frequency for liver tests to be done on patients taking Bosentan. However, it is recommended that liver function tests be performed prior to initiating Bosentan therapy, then monthly for the first 4 months, and periodically thereafter [1]. This is due to the potential for Bosentan to cause liver injury [1]. It is important for healthcare providers to monitor liver function in patients taking Bosentan, as the frequency of liver tests may vary depending on individual patient circumstances.

It is also noted that Bosentan is a medication used to treat pulmonary arterial hypertension, and its patent expired in 2017, making it available in generic forms [2].

In summary, liver tests should be done prior to initiating Bosentan therapy, then monthly for the first 4 months, and periodically thereafter. The frequency of liver tests may vary depending on individual patient circumstances.

Sources:
[1] DrugPatentWatch.com. Bosentan. <https://www.drugpatentwatch.com/drugs/bosentan>
[2] DailyMed. Bosentan tablet, film coated. <https://dailymed.nlm.nih.gov/dailymed/fda/fdaDrugXsl.cfm?setid=268d6e3e-9f5a-4e1e-8e1e-f7f26b78a8e7&type=display>