After reviewing the provided sources, it appears that the usage modification of Lipitor, a medication commonly prescribed to lower cholesterol levels, may have been influenced by several factors.

Firstly, the patent for Lipitor expired in 2011 [1]. This expiration allowed for the production of generic versions of the drug, which could have impacted its usage as more affordable alternatives became available.

Secondly, the side effects associated with Lipitor, specifically Atorvastatin, may have triggered a modification in its usage. Common side effects include muscle pain, digestive problems, and increased risk of diabetes [2]. Severe side effects, although rare, include liver damage and kidney failure [2].

Lastly, a 2012 study published in the Journal of General Internal Medicine found that statins, including Lipitor, offer less benefit to women than men [3]. This could have influenced prescribing practices and patient adherence to the medication.

In conclusion, the usage modification of Lipitor could have been triggered by the expiration of its patent, the side effects associated with the drug, and new research indicating less benefit for women.

Sources:
[1] https://www.drugpatentwatch.com/p/tradename/LIPITOR
[2] https://www.nhs.uk/medicines/atorvastatin/side-effects-of-atorvastatin/
[3] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3016087/