Lipitor, also known as atorvastatin, is a medication used to lower cholesterol levels and reduce the risk of heart disease. According to the Drug Patent Watch [1], the patent for Lipitor expired in 2011, which means that generic versions of the drug are now available. However, the expiration of the patent does not necessarily mean that the drug has lost its potency.

The Harvard Health Blog [2] explains that drug expiration dates are not always an accurate indicator of a medication's potency. The expiration date on a drug indicates the date until which the drug manufacturer can guarantee the drug's full potency and safety. After the expiration date, the drug may still be effective, but its potency cannot be guaranteed. The article also notes that the expiration date is based on the assumption that the drug will be stored in a cool, dry place. If the drug is exposed to heat, light, or moisture, its potency may degrade more quickly.

A study published in the Journal of Pharmaceutical Sciences [3] tested the potency of expired atorvastatin tablets and found that the drug was still effective up to four years after its expiration date. However, the study also noted that the potency of the drug decreased over time, and some tablets had lost up to 10% of their potency after four years.

In summary, while the expiration date on Lipitor may indicate the date until which the drug manufacturer can guarantee the drug's full potency and safety, the drug may still be effective beyond that date. The potency of the drug may decrease over time, especially if it is exposed to heat, light, or moisture. It is always best to check with a healthcare provider before taking expired medication.

Sources:
[1] https://www.drugpatentwatch.com/p/tradename/LIPITOR
[2] https://www.health.harvard.edu/staying-healthy/drug-expiration-dates-do-they-mean-anything
[3] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4799528/