Last month, an updated list of medications affected by grapefruit was released. Some drugs that have a narrow therapeutic range, where having a bit too much or too little can have serious consequences and require vigilance with regard to grapefruit, said Patrick McDonnell, clinical professor of pharmacy practice at Temple. Still, most patients suffering adverse reactions are consuming large amounts of grapefruit. “There's a difference between an occasional section of grapefruit and someone drinking 16 ounces of grapefruit juice a day,” he said.