A perennial with simple or branched square stems, sometimes creeping. Flowers in tight, cylindrical spikes, blue, lavender, or violet, the upper lip hooded, covering the stamens, the lower lip 3-lobed with the center one fringed. Flowers are subtended by tiny leaflike bracts that are often purple-tinged. Blooms May-September. Leaves opposite, ovate-lanceolate, on petioles, to 4 inches long, often with shallow teeth and wavy margins.
Similar species: Two subspecies grow in Missouri, one native and one introduced. P. vulgaris var. vulgaris, common self-heal, is a native of Europe introduced throughout the Northern Hemisphere. P. vulgaris var. lanceolata, lance-leaved self-heal, is native to North America. It has narrower leaves that taper (are not rounded) at the base.