Bioavailability: Understanding Ketamine Absorption by Route

Bioavailability is the percentage of an administered drug that reaches the bloodstream in active form and becomes available to produce a therapeutic effect. Intravenous administration is defined as 100% bioavailable by convention, since the drug goes directly into circulation. Every other route falls somewhere below that mark.

For ketamine, this matters because the drug is delivered through several different routes, including IV infusion, intramuscular (IM) injection, intranasal spray, sublingual troches, oral capsules, and occasionally rectal administration. Each route has a distinct bioavailability profile, which is a major reason doses, onset times, and the intensity of effects vary so much between delivery methods. This guide breaks down ketamine bioavailability by route and explains what the numbers mean for dosing and clinical decisions.

Why Bioavailability Varies by Route

Any route other than IV requires the drug to be absorbed from the administration site into the bloodstream before it can act. Several factors reduce how much of the original dose ends up circulating in active form.

First-Pass Metabolism

When ketamine is swallowed, it passes through the gastrointestinal tract and liver before reaching general circulation. Liver enzymes metabolize a large portion of the drug during this pass, a process covered in more detail in our guide to how ketamine works. This is the main reason oral bioavailability is lower than injected or mucosal routes.

Absorption Barriers

Mucosal membranes in the nose, mouth, or rectum, along with muscle tissue, allow drug absorption but at variable efficiency depending on blood flow, membrane permeability, and how long the drug stays in contact with the tissue.

Degradation and Incomplete Absorption

Some drug is broken down by enzymes at the absorption site before it reaches the bloodstream, and not all of the administered dose is absorbed at all. A portion can remain at the administration site or be cleared before absorption finishes.

Ketamine Bioavailability by Route

Intravenous (IV): 100%

IV administration is 100% bioavailable by definition, because the drug enters the bloodstream directly. This makes IV ketamine the most predictable and precise method of delivery, with immediate onset and the ability to titrate the dose in real time during infusion.

Intramuscular (IM): About 93%

IM injection delivers ketamine into muscle tissue, where it is absorbed quickly through surrounding capillaries. At roughly 93% bioavailability, IM is the most bioavailable non-IV route, with onset in about 3-5 minutes. It is fast and reliable, though somewhat less controllable than a continuous IV infusion.

Intranasal: 25-50%

Intranasal administration delivers ketamine through the nasal mucosa. Bioavailability ranges from 25% to 50%, with variability driven by nasal congestion, spray technique, and the specific formulation. Both compounded racemic ketamine sprays and the FDA-approved esketamine (Spravato) use this route; see our comparison of racemic ketamine vs. esketamine for how the two differ. Onset typically occurs within 5-15 minutes.

Sublingual: 25-35%

Sublingual administration places ketamine, usually as a troche or lozenge, under the tongue, where it absorbs through the oral mucosa. Bioavailability is approximately 25-35%, depending on how long the dose stays in contact with the mucosa and whether saliva is swallowed. Any portion that is swallowed is subject to first-pass metabolism, which lowers its contribution to overall bioavailability.

Oral (Swallowed): 17-24%

Oral ketamine has the lowest bioavailability of the commonly used routes, at approximately 17-24%. Extensive first-pass metabolism in the liver converts much of the dose to norketamine before it reaches systemic circulation. Norketamine itself has pharmacological activity, though, and may contribute to the therapeutic effect, which complicates a simple bioavailability comparison.

Rectal: 25-30%

Rectal administration is used less often but provides bioavailability of approximately 25-30%. It partially avoids first-pass metabolism because the lower rectal veins drain into systemic circulation rather than the portal system. This route is occasionally used in pediatric or palliative care settings.

Clinical Significance

Dose Equivalence

Bioavailability is central to calculating equivalent doses across routes. A 0.5 mg/kg IV dose, the standard research dose studied for depression, delivers the full 0.5 mg/kg into the bloodstream. To reach a roughly comparable systemic exposure by the sublingual route, at about 30% bioavailability, the administered dose would need to be closer to 1.5-1.7 mg/kg. These calculations are approximate: absorption rate, peak plasma concentration, and duration of effect also differ by route, so equal total absorption does not guarantee an equal clinical effect. Our guide to ketamine treatment methods covers how these dosing differences play out across delivery methods.

Onset and Duration

Routes with higher bioavailability generally produce faster onset and more intense peak effects, while lower-bioavailability routes tend to produce a more gradual onset and a longer, lower-intensity experience. This is one reason IV and IM routes are typically chosen when rapid, intense effects are the goal, while sublingual and oral routes may be selected for a gentler, more sustained treatment.

Individual Variability

Bioavailability is not fixed for a given route. It varies between individuals based on body composition, mucosal integrity, liver function, genetic differences in metabolizing enzymes, concurrent medications, and technique of administration. This variability is especially relevant for non-IV routes and is a key reason dosing should be individualized under clinical supervision.

References

StatPearlsKetamine, National Library of Medicine, covers ketamine pharmacokinetics and bioavailability across administration routes.

MedlinePlusKetamine Injection, National Library of Medicine drug information on ketamine administration and pharmacokinetics.

"Ketamine Pharmacology: An Update," a National Institutes of Health review of ketamine pharmacology covering absorption, distribution, and route-dependent bioavailability.