The human immunodeficiency virus (HIV) is one of the greatest current worldwide health threats. With the difficulties faced from the adverse effects of drugs used to treat HIV and the emergence of drug resistance, a new drug or medicinal herb is urgently needed. Previous study by the Department of Medical Science, Ministry of Public Health, Thailand, has demonstrated that the SN-1 extracts of the Thai medicinal herb “Khailung” (Phyllanthus sp) is a potent inhibitor of the enzyme HIV-1 reversetranscriptase in vitro. We conducted a study comparing an oral formulation of a 500 mg/day dose of SN-1 extract given for 14 days, and an oral formulation of SN-1 extract 1,000 mg/day for 14 days, to determine safety and tolerability. The study population consisted of 16 healthy Thai volunteers residing in Bangkok. Routine laboratory and physical examinations were performed every day during drug administration, and weekly during follow-up for 28 days after final drug administration, to observe any adverse effects. The results showed no finding of clinically significant abnormalities or changes from the baseline for routine laboratory, vital signs and body weights compared on anyfollow-up day. In addition, no clinical adverse reactions or drug-related effects were observed during the study. We conclude that the oral formulation of SN-1 extract was well-tolerated. The clinical pharmacology of the SN-1 extract should be further studied to explore the potential for using this drug to treat HIV-1.