Intraperitoneal

Intraperitoneal injection (also written intraperitoneally, often abbreviated IP) delivers a substance directly into the peritoneal cavity — the fluid-filled space enclosed by the membrane that lines the abdominal wall and wraps the internal organs. Rather than crossing the gut wall, a compound enters the body through the peritoneal membrane and is absorbed into the blood supply, eventually reaching the liver before the systemic circulation.

In clinical medicine, the route appears in specialised contexts: intraperitoneal chemotherapy for cancers confined to the abdominal cavity, and peritoneal dialysis for kidney failure. Its widest use, however, is in preclinical pharmacology research, where it is a standard method for dosing laboratory rodents because it is quick to perform and produces consistent results.

Onset & absorption

Absorption from the peritoneal cavity is faster than from the gastrointestinal tract, because the substance bypasses gastric emptying entirely. Effects arrive more quickly than after an oral dose, but the rise is shallower and less abrupt than intravenous injection, which places a compound directly into the bloodstream.

The peritoneal membrane allows a broad range of molecules to pass, and absorption continues over a longer window than a bolus intravenous dose would permit. In rodent research, this produces a relatively consistent blood-level curve — which is partly why the route became a laboratory standard.

Because much of the absorbed compound drains through the portal vein, substances that undergo significant hepatic metabolism may be partially processed before reaching systemic circulation. This differs from oral dosing, which also involves intestinal-wall metabolism, but it is not equivalent to bypassing the liver entirely.

Harm reduction

The peritoneal cavity is not a physiological entry point for foreign substances. Introducing a needle or cannula carries a direct risk of puncturing the intestines, the bladder, or nearby vascular structures — injuries that typically require surgical management.

Bacterial contamination of the cavity causes peritonitis, an infection of the peritoneal lining that can become life-threatening rapidly. This risk applies regardless of the compound administered; sterile technique reduces but does not eliminate it.

Intraperitoneal procedures in human medicine are performed by trained clinicians under controlled conditions. The route is not practical for unsupervised self-administration, and the anatomical risks do not diminish with familiarity outside a clinical environment.

What varies by substance

How completely a compound is absorbed from the peritoneal cavity depends on its molecular size, its solubility in fat versus water, and how strongly it binds to proteins in peritoneal fluid. These properties vary widely across substances and cannot be predicted without compound-specific data.

Critically, the pharmacokinetics observed in rodent intraperitoneal studies do not transfer directly to human routes of administration. A compound that reliably produces effects in mice via this route may be poorly absorbed when swallowed, or may be metabolised very differently in the human liver. Activity seen in animal IP studies should not be assumed to hold for any human exposure pathway.

For any substance, the route used in the underlying research — and what that implies for human pharmacology — is noted on that substance's record alongside its sources.