The present disclosure is directed to chemically-stable and pharmaceutically acceptable sildenafil oral spray formulations for the treatment of diseases such as pulmonary arterial hypertension and/or SSRI-induced sexual dysfunction, wherein the oral spray formulation has a pH of about 1.5 to less than 3.0. The present disclosure is also directed to methods for treating diseases such as pulmonary arterial hypertension and/or SSRI-induced sexual dysfunction.