In the preclinical phase, preclinical or non-clinical studies take place. They study the chemical and pharmaceutical properties of promising molecules (drug candidates) and their possible toxic and carcinogenic properties. This is done at first in vitro, in test tubes or Petri dishes in the laboratory, on human cell cultures. Afterwards, the efficacy and safety of the drug candidates are also studied in laboratory animals. Only when preclinical studies show that a drug candidate is effective and sufficiently safe can human clinical studies begin. The preclinical phase usually lasts several years.