According to the FDA its got to be one of these---
Some examples of reasons for clinical hold are shown below:
Product Quality:
Product has an impurity profile indicative of a potential health hazard or an impurity profile insufficiently defined to assess health hazard
Product cannot remain chemically stable throughout the testing program
Pharmacology and Toxicology:
Data from animal studies are not sufficient to support the anticipated exposure (dose, route of administration, and duration) for the proposed clinical trial
Poor quality non-GLP toxicology studies with difficult to interpret results and a safety signal that has not been sufficiently studied are proposed as pivotal toxicology studies to support use of the product in humans
Clinical:
Previously observed toxicities of the product are not addressed by the proposed safety assessments in a clinical protocol