Definition
Bioavailability is a pharmacological term that describes the proportion of an administered drug that reaches the systemic circulation (the bloodstream) in an active form and is therefore available to exert its therapeutic effects. It is expressed as a percentage, with intravenous (IV) administration defined as 100% bioavailable by convention, since the drug is delivered directly into the bloodstream.
Bioavailability is a critical concept in ketamine therapy because ketamine is administered through multiple routes — each with a substantially different bioavailability profile. The route of administration directly affects the dose required, the onset and duration of effects, and the intensity of the therapeutic and psychoactive experience.
Why Bioavailability Varies by Route
When a drug is administered by any route other than IV, it must be absorbed from the site of administration into the bloodstream. Several factors reduce the amount of active drug that reaches systemic circulation:
- First-pass metabolism — When a drug is swallowed, it passes through the gastrointestinal tract and liver before reaching the general circulation. Enzymes in the liver metabolize a portion of the drug — a process explored in detail in our ketamine pharmacology guide, reducing the amount of active compound available. This is the primary reason oral bioavailability is lower than parenteral routes.
- Absorption barriers — Mucosal membranes (in the nose, mouth, or rectum) and muscle tissue allow drug absorption but with variable efficiency depending on blood flow, membrane permeability, and contact time.
- Degradation — Some drug may be degraded by enzymes at the absorption site before reaching the bloodstream.
- Incomplete absorption — Not all of the administered drug may be absorbed; some may remain at the administration site or be cleared before absorption is complete.
Ketamine Bioavailability by Route
Intravenous (IV) — 100%
By definition, IV administration provides 100% bioavailability 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) — ~93%
IM injection delivers ketamine into muscle tissue, where it is rapidly absorbed through surrounding capillaries. With approximately 93% bioavailability, IM is the most bioavailable non-IV route. Absorption is fast (onset in 3-5 minutes) and reliable, though somewhat less controllable than IV infusion.
Intranasal — ~25-50%
Intranasal administration delivers ketamine through the nasal mucosa. Bioavailability ranges from 25% to 50%, with variability influenced by nasal congestion, technique, and the specific formulation. Both compounded racemic ketamine sprays and FDA-approved esketamine (Spravato) use this route. Onset occurs within 5-15 minutes.
Sublingual — ~25-35%
Sublingual administration involves placing ketamine (typically as a troche or lozenge) under the tongue, where it is absorbed through the oral mucosa. Bioavailability is approximately 25-35%, depending on the duration of mucosal contact and whether saliva is swallowed. Any portion that is swallowed is subject to first-pass metabolism, reducing its contribution to systemic bioavailability.
Oral (Swallowed) — ~17-24%
Oral ketamine has the lowest bioavailability of commonly used routes, at approximately 17-24%. Extensive first-pass metabolism in the liver converts much of the ketamine to norketamine before it reaches the systemic circulation. However, norketamine itself has pharmacological activity and may contribute to the therapeutic effect, complicating the simple bioavailability comparison.
Rectal — ~25-30%
Rectal administration, though less commonly used, provides bioavailability of approximately 25-30%. It partially avoids first-pass metabolism because the lower rectal veins drain into the systemic circulation rather than the portal system. This route is occasionally used in pediatric or palliative care settings.
Clinical Significance
Dose Equivalence
Understanding bioavailability is essential for calculating equivalent doses across different routes. A 0.5 mg/kg IV dose of ketamine — the standard research dose for depression — delivers all 0.5 mg/kg to the bloodstream. To achieve a roughly equivalent systemic exposure via the sublingual route (at ~30% bioavailability), the administered dose would need to be approximately 1.5-1.7 mg/kg.
However, dose equivalence calculations are approximate. The rate of absorption, peak plasma concentration, and duration of effect also differ across routes, meaning that equivalent total absorption does not necessarily produce equivalent clinical effects.
Onset and Duration
Routes with higher bioavailability generally produce faster onset and more intense peak effects, while routes with lower bioavailability tend to produce more gradual onset and a more prolonged, lower-intensity experience. This pharmacokinetic profile influences clinical decision-making — IV and IM routes are preferred when rapid, intense effects are desired, while sublingual and oral routes may be chosen for gentler, more sustained treatment.
Individual Variability
Bioavailability is not fixed for any given route — it varies among individuals based on factors such as body composition, mucosal integrity, liver function, genetic variations in metabolizing enzymes, concurrent medications, and technique of administration. This variability is particularly relevant for non-IV routes and underscores the importance of individualized dosing under clinical supervision.
Key Takeaways
- Bioavailability measures the percentage of an administered drug that reaches the bloodstream in active form
- IV ketamine is 100% bioavailable; IM is approximately 93%; intranasal is 25-50%; sublingual is 25-35%; oral is 17-24%
- The route of administration determines the dose required, the onset and duration of effects, and the intensity of the experience
- Bioavailability varies among individuals and is influenced by multiple physiological factors
- Understanding bioavailability is essential for appropriate dosing and for comparing the clinical effects of different ketamine formulations. For more on dosing considerations, see the pharmacokinetics overview
References
- StatPearls: Ketamine — Clinical reference covering ketamine pharmacokinetics and bioavailability across administration routes
- Ketamine Pharmacology: An Update — NIH review of ketamine pharmacology including absorption, distribution, and route-dependent bioavailability
- MedlinePlus: Ketamine Injection — National Library of Medicine drug information on ketamine administration and pharmacokinetics
Share