According to Drug Patent Watch [1], Lipitor (atorvastatin) is a prescription medication used to lower cholesterol levels and reduce the risk of heart disease. The patent for Lipitor expired in 2011, and since then, generic versions of the drug have been available.

Regarding the expiration date of drugs, Harvard Health Publishing [2] explains that drug expiration dates are typically set by manufacturers based on their testing of the drug's stability. The expiration date is the date after which the manufacturer can no longer guarantee full potency and safety of the drug. However, this does not necessarily mean that the drug is no longer effective or safe to use.

In fact, a study by the U.S. Department of Defense [3] found that many drugs remain effective even years after their expiration dates. The study tested the potency of drugs stored under ideal conditions (cool, dry, and dark) and found that most drugs were still effective up to several years after their expiration dates.

However, it's important to note that not all drugs will remain effective or safe after their expiration dates. As the Harvard article explains, certain drugs, such as insulin and liquid antibiotics, are more susceptible to degradation and may be less effective or even harmful if used after their expiration dates.

In summary, while Lipitor may still be effective after its expiration date, it's important to use caution when taking any expired medication. It's always best to check with a healthcare provider before taking any expired medication or medication that has been stored improperly.

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.npr.org/sections/health-shots/2017/07/18/537257884/that-drug-expiration-date-may-be-more-myth-than-fact