Background: Oral S-ketamine (S-KETPO) is explored as an alternative for intravenous maintenance treatment (S-KETIV) in treatment-resistant depression (TRD). Its pharmacokinetic profile is expected to be significantly altered due to first-pass metabolism, potentially resulting in a distinct pharmacodynamic and safety profile. Aims: This study aimed to systematically characterize the pharmacokinetic, pharmacodynamic and safety profiles of S-KETPO in healthy participants, and to compare these to a previously demonstrated therapeutic S-KETIV dose in TRD. Methods: In a randomized, double-blind, placebo-controlled, double-dummy, 4-way cross-over study, 17 healthy volunteers received a single dose of S-KETPO 0.20 mg/kg and 0.45 mg/kg, S-KETIV 0.40mg/kg and placebo. Plasma concentrations of S-KET and its active metabolite norketamine (S-NOR) and S-hydroxynorketamine (S-HNK) were measured, safety assessments were conducted up to 24 hours post-dose, and central nervous system effects were evaluated up to 6 hours post-dose. Results: Mean peak plasma concentrations were 9.80, 62.00 and 29.50 ng/mL (S-KET); 22.70, 127.00 and 62.10 ng/mL (S-NOR); and 146.00, 55.20 and 32.20 ng/mL (S-HNK) for S-KETPO 0.20 mg/kg, S-KETPO 0.45 mg/kg and S-KETIV, respectively. S-NOR and S-HNK exposure was comparable between S-KETPO 0.20 mg/kg and S-KETIV. Absolute bioavailability of S-KETPO was 9-12%. S-KETIV produced sedative, psychomotor and psychotomimetic effects, whereas S-KETPO 0.45 mg/kg demonstrated inconsistent effects on vigilance and arousal. S-KETPO 0.20 mg/kg lacked PD effects. Safety was comparable across treatments, although S-KETPO appeared better tolerated. Discussion and Conclusion: Oral administration meaningfully alters the pharmacological profile of S-ketamine in humans, which should be considered in dose selection, study design, and interpretation of efficacy studies in depression.