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Inclusion Criteria: 1. Willingness to provide written informed consent prior to participate in the study. 2. Healthy Thai male subjects are between 18 to 55 years of age. 3. The Body Mass Index (BMI) ranges from 18.5 to 30 kg/m2. 4. Comprehensive of the nature and purpose of the study and compliance with the requirement of the entire protocol and allow investigators to draw 7 mL of blood for monitoring subjects' safety after the completion of the study. 5. Absence of significant diseases or clinically significant abnormal laboratory values on the laboratory evaluations, medical history or surgery during the screening. Some of the laboratory values e.g. Complete blood count etc. that out of the normal range will be carefully considered by physician Exclusion Criteria: 1. History or evidence of allergy or hypersensitivity to Sildenafil any related drugs or any of the excipients of this product. 2. Subject with B.P. is Systolic B.P \< 100, ≥140 mm/Hg, Diastolic B.P \< 70, ≥90 mm/Hg or pulse rate \> 100 beats per minute. For study periods, BP evaluation criteria for AE will be \< 90, ≥140 mm/Hg, Diastolic B.P \< 60, ≥90 mm/Hg. 3. Serum bilirubin greater than 1.5 times the upper limit of reference range (ULRR).\* 4. Serum creatinine greater than 1.5 times the upper limit of reference range (ULRR).\* 5. Alanine amino transferase (ALT) or aspartate amino transferase (AST) greater than 2 times the upper limit of reference range (ULRR).\* 6. Positive of hepatitis B or C virus or HIV. 7. Have more than one abnormal EKG, which is considered as clinically significant. \* 8. History or evidence of heart, renal, hepatic disease, pulmonary obstructive disease, bronchial asthma, hypertension, glaucoma, sickle cell anemia, multiple myeloma, or leukemia. 9. History of a myocardial infarction, stroke, or life-threatening arrhythmia within the last 6 months. 10. History of cardiac failure or coronary artery disease causing unstable angina. 11. History or evidence of gastrointestinal disorder likely to influence drug absorption or previous GI surgery other than appendectomy. 12. Any major illness in the past 3 months or any significant ongoing chronic medical illness. 13. History of psychiatric disorder. 14. History or presence of major active bleeding or abnormal bleeding due to hemorrhoids, epistaxis, gastrointestinal ulcer or other symptoms in past 1 month before the study drug administration and until the completion of the study. 15. History of regular alcohol consumption exceeding 14 drinks/week for males (1 drink = 5 ounces (150 mL) of wine or 12 ounces (360 mL) of beer or 1.5 ounces (45 mL) of hard liquor) and cannot stop at least 2 days before the study drug administration and until the completion of each period of the study. 16. History of usually smoking (more than 10 cigarettes per day within past 1 year), if moderate smokers (less than 10 cigarettes per day) cannot stop at least 7 days before the study drug administration and until the completion of the study. 17. High caffeine consumption (more than 5 cups of coffee or tea per day) and cannot stop at least 2 days before the study drug administration and until the completion of each period of the study. 18. History of grapefruit, pomelo or grapefruit products consumption and cannot stop at least 7 days before the study drug administration and until the completion of the study. 19. Positive drug abused test in urine (Benzodiazepines, Marijuana (THC), Methamphetamine, Cocaine and Opioids). 20. Receipt of any prescription drug therapy within 14 days or 5 half-lives (whichever longer) preceding the first dose of study medication or over-the-counter (OTC) drugs or herbal medicines/food supplement within 7 days or hormonal methods of contraception within 28 days (Depo-Provera must be discontinued at least 6 months) prior to receiving the first dose of study medication. 21. History of difficulty in accessibility of veins in left and right arm. 22. Blood donation (one unit or 450 mL) within the past 3 months before the study. 23. Participation in any clinical study within the past 3 months before the study. 24. Subjects who are unwilling or unable to comply with the lifestyle guidelines described in this protocol. (\* Depend on decision of principal investigator and/or clinical investigator)