Antiallergic drugs have various actions against allergy-associated cells and molecules as well as antihistamic properties. We studied the effects of antiallergics on the serum levels of substance P. Patients with atopic dermatitis were treated with one of four oral H1-antagonists for 14 days, and the serum level of substance P was measured before and after treatment in parallel with several atopic severity markers. Olopatadine significantly decreased the substance P level. This is in accordance with its known downmodulatory effect on tachykinin release. In contrast, cetiridine and fexofenadine unexpectedly increased the substance P level. In patients administered cetiridine, the blood severity markers for atopic dermatitis, including lactate dehydrogenase, eosinophil number, and the soluble forms of IL-2R, E-selectin, VCAM-1 and ICAM-1 were reduced after the treatment. Therefore, the elevation of SP was unrelated to the deterioration of atopic dermatitis but rather associated with improvement. Our study suggests that antiallergics can be divided into substance P-increasing and -decreasing types and raises the possibility that the increment of substance P by the former type is caused by the competitive occupation of substance P receptors.