While secondary compounds are produced by plants in low abundance, these bioactive compounds are essential to human survival for their medicinal applications. These same compounds are crucial to plants, having evolved as defense mechanisms against herbivory. Chief among the theories of plant responses to herbivory, the Optimal Defense Theory (ODT) hypothesizes that plants will allocate defenses in direct proportion to the risk of a particular plant part to herbivory and the value of that part in terms of loss of fitness to the entire plant. Through insect damage assessments and antimicrobial assays, this study investigates the correlation between herbivory and medicinal activity and whether or not the within-plant ODT is followed in three ethnobotanically useful Malvaceae species, Thespesia populnea, Hibiscus tiliaceus and Hibiscus rosa-sinensis. All three plant species demonstrated an inverse relationship between herbivory and medicinal activity. Variation in secondary composition data from both insect damage assessment and antimicrobial tests supported the ODT.