The oil is distilled from freshly cut, partially dried plant tops. These are cut just before the plants come into flower so the oil will have the best balance of fragrant constituents. If allowed to over-mature, the quantity and quality of the resulting oil will suffer, developing off-aromas that can be sharp, bitter and overly menthol-scented.The primary constituent of peppermint oil is menthol, which causes a physical reaction when inhaled or applied to the skin. Menthol produces an immediate and pronounced sensation of coolness which the body reacts to quite strongly, producing its own warming effect as blood flows into the area of application. This physical sensation impresses the senses as a medicinal effect and is partially responsible for peppermint's long history of use as medicine. Today menthol is often found in sports creams and chest rubs.