Drug-induced gingival enlargement (GE) is a frequent adverse effect observed in patients treated with anticonvulsant, immunosuppressant, and some antihypertensive medications-the antiepileptic phenytoin being the main drug associated with GE due to its high incidence (around 50%). The molecular mechanisms behind drug-induced gingival overgrowth are still unknown. By reverse transcription polymerase chain reaction, we demonstrate that the calcium-permeable ion channels TRPA1, TRPV1, and its capsaicin-insensitive isoform TRPV1b are expressed in human gingival fibroblasts (HGFs), the most abundant cellular type in periodontal tissue. Cultured HGFs responded with intracellular calcium elevations to phenytoin and to the canonical TRPA1 agonist allyl isothiocyanate. Application of phenytoin activated a nonselective cationic current in HGFs with a typical signature for TRPA1 channels. Moreover, this activation was blocked by HC030031, a specific TRPA1 blocker. Similarly, the use of shRNAs against hTRPA1 in HGFs reduced TRPA1 expression and activation by phenytoin. In addition, we show that phenytoin increased intracellular calcium levels in cells transfected with mouse or human TRPA1 channels. Responses to phenytoin were not observed in untransfected cells or cells expressing TRPM8 or TRPV1. The activation of HGFs by phenytoin was markedly reduced in the presence of antioxidant vitamins: Ascorbic acid, folic acid, and α-Tocopherol. By performing cell proliferation assays, we found that phenytoin did not augment the proliferation rate of HGFs. In contrast, alcian blue and picrosirius red staining of long-Term HGFs cultures indicated that phenytoin induces extracellular matrix accumulation of collagen. Collectively, these findings support an important role of TRPA1 channels in phenytoin-induced GE, provide insight into the pathophysiologic mechanism, and offer novel therapeutic opportunities for its treatment.