Coning may occur during dissolution of an immediate release dosage form after it disintegrates in a standard USP round bottom vessel and a mound of excipients forms, possibly entrapping drug beneath the rotating spindle.

Coning may particularly impact the dissolution of a poorly soluble drug because the drug is not being effectively exposed to the medium.
Coning occurs because the mixing beneath the paddle of a conventional USP vessel is suboptimal and a zone exists where mixing is inadequate, this typically occurs due to slow rotation (rpm) speed.

Hydrodynamics of USP I-II vessel

For biorelevant testing, it is essential that coning does not occur. Coning can adversely affect drug release which could lead to misleading interpretations and conclusions about the drug product.