Oral diseases, including caries and periodontal disease, are especially of concern to public health, where they affect a large part of the population, leading dental professionals to seek alternatives for their control. The aim of this work was to evaluate the chemical composition and antibacterial activity of the essential oil of Plectranthus amboinicus Lour (malvariço), a plant used by the local population for the treatment of diseases of the oral cavity, alone or as a mouthwash against a strain of Streptococcus mutans. The essential oil obtained from the malvariço plant, obtained from two locations, was characterized by gas chromatography and evaluated for antibiotic activity. The essential oil was also tested for antibacterial-modifying activity in combination with mouthwash to determine any synergistic or antagonistic effect. The yield of the essential oils extracted from plants from Fortaleza was higher than that extracted from Crato plants (0.25 and 0.15%, respectively). The two samples of essential oil exhibited significant differences in composition, where the major compounds, carvacrol and germacrene D, varied. Antibacterial activity in the two samples was low or not detectable. The essential oil combined with mouthwash was effective in inhibiting bacterial growth, but this result was lower than with chlorhexidine alone (p<0.05). Our results demonstrated an antagonistic effect resulting from the association between the essential oil and mouthwash, indicating that this combination must be avoided and demonstrating the necessity of more studies about these combinations.