Effect of St. John's Wort with Drugs

Why does literature on Zocor warn against taking it along with St. John's Wort?

St. John's wort affects the liver enzymes that remove certain drugs from your body. It may cause the drugs to be removed from your body more quickly, decreasing the length of time of their effect. Similarly, drug literature for many drugs warns against taking grapefruit while on the drug. Grapefruit inhibits the enzymes that remove certain drugs in the body. The drugs stay in the body longer with grapefruit, increasing their effect.