OBJECTIVE The objective of the study was to evaluate the safety and tolerability of two doses (8 mg and 25 mg once daily) of a 2:1 standardized extract of the South African medicinal plant Sceletium tortuosum (L.) N.E. Br., trademarked Zembrin,(®) in healthy adult volunteers over a three-month period. METHODS This was a randomized, double-blind, parallel-group, placebo-controlled single center study. METHODS Tiervlei Trial Centre, Karl Bremer Hospital, Bellville, Cape Town, South Africa. METHODS The study took place between February 2 and July 27, 2009. Thirty-seven healthy adults were recruited from the general population. METHODS Participants were randomized to receive either one of two doses of study medication, or an identical placebo, taken once daily for 3 months. Of the 37 subjects, 12, 12, and 13 subjects received 8 mg extract Sceletium tortuosum (Zembrin), 25 mg extract Sceletium tortuosum (Zembrin), and placebo treatment, respectively. METHODS No efficacy variables were assessed. The safety and tolerability variables comprised of vital signs, physical examination, 12-lead electrocardiogram (ECG), laboratory assessments (hematology, biochemistry, and urinalysis), and the recording of adverse events (AEs). RESULTS There were no apparent differences between the three treatments with regard to vital signs, 12-lead ECG, body weight, and physical examination from screening to the end of the 3-month treatment period. No significant changes were observed in hematology or biochemistry parameters between initial screening and the end of the study. Both doses of extract Sceletium tortuosum (Zembrin) were well-tolerated. The most commonly reported AE was headache, followed by abdominal pain and upper respiratory tract infections, all with greater incidence in the placebo group than in the treatment groups. Unsolicited positive effects on well-being were noted in patient diaries by some participants taking extract Sceletium tortuosum (Zembrin), including improved coping with stress and sleep. CONCLUSIONS Both doses of extract Sceletium tortuosum (Zembrin) (8 mg and 25 mg) were well tolerated when used by healthy human subjects once daily for 3 months.