What volume of distribution (Vd) actually represents, why it isn't a physical volume, and how it shapes the relationship between a dose in mg and a serum level in ng/dL.
Volume of distribution — usually written Vd or V — is one of the two numbers that turns a dose in milligrams into a serum concentration in ng/dL or pg/mL. The other is bioavailability. DoseCurve does not display either, because both are highly individual and outside what a pharmacokinetic visualiser can responsibly estimate. But understanding what Vd represents is the difference between reading the chart as "shape over time" and over-reading it as "this is my serum testosterone".
This is background reading, not a recommendation. The values below are illustrative — your own pharmacokinetics depend on your body composition, your liver and kidney function, your concurrent medications and a dozen other factors that only a clinician with your bloodwork can speak to.
For an injected drug that distributes evenly through the body, the relationship between total amount in the body and measured plasma concentration is:
Vd = amount in body / plasma concentration
Rearranging: if you know Vd, you can convert milligrams to ng/dL and back. A young adult given 10 mg of an intravenous drug who measures a plasma concentration of 0.1 mg/L (100 ng/mL) has an apparent Vd of 10 mg / 0.1 mg/L = 100 L. That number is bigger than the human body, which is the first clue that Vd is not a real anatomical volume.
Vd is sometimes called the apparent volume of distribution for a reason. It is the volume the drug would have to be diluted into to give the measured plasma concentration, assuming the plasma concentration is representative of the whole body. For drugs that distribute mostly into tissue — fat-soluble compounds in particular — most of the dose is sequestered outside the bloodstream, so the plasma concentration is low and the calculated Vd is artificially huge. Highly tissue-bound drugs can have a Vd in the thousands of litres.
The reverse is also true. A drug that stays in the bloodstream (large, water-soluble, highly protein-bound to albumin) gives a high plasma concentration per dose and a small Vd — often close to the plasma volume itself, around 3 L in an adult, or the extracellular fluid volume, around 14 L.
Approximate adult Vd values, drawn from clinical pharmacology references:
| Drug class | Approximate Vd | Distribution |
|---|---|---|
| Heparin | ~5 L | Plasma only |
| Aminoglycosides | ~14 L | Extracellular fluid |
| Phenytoin | ~50 L | Total body water with some tissue |
| Testosterone (free) | ~120–150 L | Wide, lipophilic |
| Lipophilic depot esters | hundreds of L | Strongly tissue-bound |
| Amiodarone | ~4000–5000 L | Extreme tissue sequestration |
These are population averages. Real values vary by individual.
Vd is not a fixed property of the molecule alone — it depends on the host body. The factors that shift it include:
The practical consequence: two people given the same dose of the same compound can land on different plasma concentrations, and the gap can be substantial.
There is a clean mathematical relationship between the three core PK parameters:
half_life = (ln(2) × Vd) / clearance
This is one of the most useful equations in pharmacology because it tells you what actually drives the half-life you see. A long half-life can come from a large Vd (lots of tissue storage) or a low clearance (slow elimination), and the practical implication of each is different. A long half-life driven by tissue storage typically also has a long redistribution tail; a long half-life driven by slow clearance does not.
This is also why depot esters have such long half-lives despite the parent hormone being cleared in minutes: the rate-limiting step is release from the oil depot, which keeps an effective amount in the body for days to weeks. The DoseCurve model collapses this into a single half-life term — useful for visualising shape, insufficient for predicting absolute serum levels.
A pharmacokinetic visualiser could, in principle, multiply mg-remaining by a population-average Vd and bioavailability and show ng/dL. We deliberately do not, for three reasons:
If you want a calibrated serum estimate, the most reliable workflow is: model the shape with DoseCurve, get bloodwork at a known time point relative to your last injection, and use the measured value to anchor your personal sense of where the curve sits in absolute terms.