Rhinosinusitis involves complex pathophysiological cascades beginning with viral mucosal invasion that triggers epithelial damage, inflammatory cytokine release, and mucociliary dysfunction. Sinus ostia obstruction creates an anaerobic environment with reduced oxygen tension and pH changes that promote bacterial colonization. Viral-bacterial synergy occurs through multiple mechanisms: direct viral damage to ciliated epithelium, upregulation of bacterial adhesion receptors on nasal epithelial cells, disruption of mucosal immune barriers, and alteration of the sinonasal microbiome. The transition from viral to bacterial sinusitis involves an aerobic phase (S. pneumoniae, H. influenzae, M. catarrhalis) potentially progressing to an anaerobic phase (Peptostreptococcus, Prevotella, Fusobacterium, Bacteroides fragilis, Porphyromonas species). Phenylephrine acts as a selective alpha-1 agonist, producing vasoconstriction of nasal mucosal blood vessels, decreasing hydrostatic capillary pressure, and reducing mucosal blood volume. However, repeated stimulation causes receptor desensitization, compensatory vasodilation, increased vascular permeability, edema, and inflammatory cell infiltration—the pathophysiology of rhinitis medicamentosa. The clinician must differentiate viral from bacterial etiology, identify complications, prescribe evidence-based therapy, and manage antibiotic stewardship.