Preclinical tests assess the toxicity of a drug and examine its potential effects on the human body. The tests are conducted in-vitro (within the glass- test tubes, Petri dishes etc) and in-vivo (within the living - animal testing). These tests are required by pharmaceutical regulators to ensure that a drug is safe for human use prior to beginning human clinical trials.
In-vivo trials may involve Pharmacokinetic (PK) and Pharmacodynamic (PD) studies, to begin to ascertain the drug's mechanism of action. PK and PD trials study what a body does to the drug and what a drug does to a body, respectively.
Also investigated through pre-clinical trials is a drugs No Observable Effect Level (NOEL). This forms the starting point for subsequent trials and is the level at which the drug is dosed in amounts too small to have any measurable effect.