In vivo

In vivo (Latin: 'within the living') describes research conducted inside a whole, living organism — most often a laboratory animal such as a rat or mouse, though the term applies equally to studies in humans. It contrasts with in vitro work, which takes place in isolated cells, tissue preparations, or chemical systems outside any organism.

In the hierarchy of evidence a reader will encounter on these pages, in-vivo findings sit between cell-based assays and human clinical data. An intact body brings its own metabolism, circulatory system, blood-brain barrier, and immune response to bear — factors that can sharpen, blunt, or redirect an effect that looked straightforward in a dish.

What this design can establish

A well-run in-vivo study can establish that a compound produces a measurable effect in an organism and characterise how that effect changes with dose. It can capture behavioural and physiological responses — changes in body temperature, movement, organ function, hormone levels — that are invisible in cell culture.

It can also trace how a substance moves through the body: absorption rate, tissue distribution, metabolic transformation, and clearance. These pharmacokinetic properties depend on the interplay of enzymes, transport proteins, and organ systems; they cannot be reliably estimated from isolated tissue alone.

For substances with limited human data, in-vivo animal findings often represent the most detailed pharmacological picture available. An absent in-vivo study is a meaningful gap in the record, not a neutral baseline.

What it cannot

An in-vivo finding in an animal establishes that something happened in that animal, under those conditions. It does not establish that the same event occurs in a human.

Species differences in metabolism are large and frequently decisive. A compound cleared rapidly in a mouse may accumulate to much higher levels in a human; one that reliably produces a behavioural effect in a rat may produce no detectable effect in a person because the relevant receptor population is distributed differently, or because the human metabolite differs from the rodent's in its own activity.

The inference a reader is most tempted to draw — that animal evidence confirms a human effect — is the one in-vivo data cannot support on its own. Animal findings establish biological plausibility and motivate further study; they do not substitute for it. When a substance page cites only in-vivo evidence, that is a statement about the current shape of the record, not a confirmed human effect.