It contains an irritant which causes intense discomfort to the lips, mouth, and throat. The acridity is caused in part by microscopic needle like raphides of calcium oxalate and probably by another chemical. It must be processed by cooking,soaking or fermenting.
Our garden is surrounded with herbs, like this basil. I love the aroma of freshly cut basil around the kitchen. I usually cut a bunch of herbs and hang them on our pot-rack in the kitchen.