Grapefruit juice can affect the effectiveness of Lipitor, a cholesterol-lowering medication. Lipitor contains atorvastatin, which is metabolized by the CYP3A4 enzyme in the liver. Grapefruit juice contains compounds called furanocoumarins that inhibit the activity of this enzyme, which can lead to higher levels of atorvastatin in the bloodstream [2]. This can increase the risk of side effects, such as muscle pain and liver damage, and reduce the effectiveness of the medication [3]. It is recommended that people taking Lipitor avoid consuming grapefruit juice or talk to their doctor about alternative medications [2][3]. Other medications that are metabolized by the CYP3A4 enzyme may also be affected by grapefruit juice, so it is important to check with a healthcare provider before consuming grapefruit juice while taking any medication [2].

Sources:
[2] https://www.health.harvard.edu/heart-health/grapefruit-juice-and-statins
[3] https://www.verywellhealth.com/can-i-take-lipitor-or-zocor-with-grapefruit-juice-1124045