What effect does grapefruit juice have on CYP3A4?

Grapefruit juice is known for its effect on the cytochrome P450 enzyme system, particularly CYP3A4. This enzyme is crucial for the metabolism of many medications. When grapefruit juice is consumed, it contains compounds that inhibit the activity of CYP3A4 in the intestinal walls. This inhibition decreases the metabolic breakdown of drugs that are CYP3A4 substrates, leading to increased levels of these drugs in the bloodstream.

As a result, the correct understanding is that the inhibition of CYP3A4 leads to a reduction in the metabolism of medications, which can cause higher drug concentrations in the body, potentially leading to increased drug effects or toxicity. Therefore, the function of grapefruit juice in this context is to inhibit, not enhance, the activity of CYP3A4, which aligns with the answer provided. The other options, implying stimulation of activity or reduced drug absorption, do not reflect the established interaction between grapefruit juice and CYP3A4.