Lipitor is a medication used to lower cholesterol levels in the body. It contains the active ingredient atorvastatin calcium and is manufactured by Pfizer [1]. The drug works by blocking an enzyme that is responsible for producing cholesterol in the liver, thereby reducing the amount of cholesterol in the bloodstream [2].

The effectiveness of Lipitor over time is not related to the drug's expiration date. According to a Harvard Health Publishing article, drug expiration dates are not always accurate and do not necessarily mean that the drug has become ineffective after that date. In fact, many drugs are still effective long after their expiration date has passed [2].

Instead, the effectiveness of Lipitor is maintained through its mechanism of action. The drug continues to block the enzyme responsible for producing cholesterol in the liver, which in turn continues to reduce the amount of cholesterol in the bloodstream. However, it is important to note that the effectiveness of Lipitor can be influenced by factors such as the patient's diet and exercise habits, as well as other medications they may be taking [3].

In terms of drug interactions, it is important to note that Lipitor can interact with other medications, including ritonavir-boosted nirmatrelvir (Paxlovid), which is used to treat COVID-19. According to the NIH COVID-19 Treatment Guidelines, concomitant use of Lipitor and Paxlovid may increase the risk of Lipitor-related adverse events, and therefore Lipitor dosing may need to be adjusted in patients taking Paxlovid [4].

Overall, the effectiveness of Lipitor is maintained through its mechanism of action, and is not related to the drug's expiration date. However, it is important for patients to be aware of potential drug interactions and to follow their healthcare provider's instructions regarding dosing and other medications.

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.medicalnewstoday.com/articles/323224
[4] https://www.covid19treatmentguidelines.nih.gov/therapies/antivirals-including-antibody-products/ritonavir-boosted-nirmatrelvir--paxlovid-/paxlovid-drug-drug-interactions/