Ketamine Half-Life by Route of Administration Explained

Ketamine's terminal elimination half-life runs about 2 to 3 hours whether it's given by intravenous (IV), intramuscular (IM), subcutaneous, or oral route, according to a widely cited 2016 pharmacokinetic review published in Clinical Pharmacokinetics. The outlier is intranasal esketamine (Spravato), the only ketamine-based medication with U.S. Food and Drug Administration (FDA) approval, whose FDA prescribing information lists a terminal half-life of 7 to 12 hours, roughly three to five times longer than IV ketamine. That gap comes down to how each formulation is absorbed and processed, not how long you'll feel a treatment's effects. Here's what the numbers mean route by route, and why half-life isn't the same measurement as onset or duration.

Half-Life Isn't the Same as Onset or Duration

Half-life describes elimination, the pace at which the body clears a drug from the bloodstream after it's been absorbed. On its own, it says nothing about how quickly a dose takes effect or how long someone feels sedated, dissociated, or notices a mood response. Those depend on absorption rate, how much drug reaches the brain, and receptor binding, factors covered in more detail in ketamine onset time by route of administration.

IV ketamine also has a distribution phase separate from its elimination half-life: plasma concentration falls fast in the first 10 to 15 minutes as the drug moves from blood into tissue, then declines more slowly over the following hours as the liver metabolizes it. IM and subcutaneous doses follow a similar elimination pattern once absorbed, though subcutaneous ketamine administration is absorbed more gradually than an IM injection, which can smooth out peak concentration without changing the underlying half-life.

Why Esketamine's Half-Life Looks So Different

Esketamine, the S-enantiomer of ketamine, was cleared by the FDA in 2019 for treatment-resistant depression when used with an oral antidepressant under direct clinical supervision. See how it compares to generic ketamine in esketamine (Spravato) and in the side-by-side breakdown at esketamine vs. racemic ketamine. Its 7 to 12 hour apparent half-life is longer than IV or IM ketamine's largely because nasal absorption is slow and incomplete, only about 48% of the dose reaches systemic circulation, so the drug keeps entering the bloodstream well after the spray is administered. Pharmacologists sometimes call this a flip-flop effect: when absorption is slower than elimination, the measured half-life reflects the absorption rate more than the body's actual clearance speed.

Norketamine and the Oral, Sublingual, and Buccal Routes

Oral ketamine has the lowest bioavailability of any route, around 16 to 20%, because it passes through the liver before reaching general circulation, a process called first-pass metabolism. That process converts much of the dose into norketamine, ketamine's primary active metabolite, before it ever reaches the bloodstream intact. Norketamine binds NMDA (N-methyl-D-aspartate) receptors more weakly than ketamine itself but has a longer half-life, so it can extend a dose's effects beyond what ketamine's own half-life would suggest. Sublingual and buccal ketamine administration partially bypass first-pass metabolism by absorbing through the mouth's mucous membranes, raising bioavailability to roughly 30% compared with oral dosing, though norketamine still contributes meaningfully to the overall effect. More detail on this metabolic pathway is in ketamine metabolites: norketamine and hydroxynorketamine.