The science of toxicology is the science of poison.
Toxicologists study the adverse effects of chemical agents on biological systems and estimates the results and hazards of these effects. Many therapeutics are potent poisons, including chemotherapy and vaccines. Botox, a therapeutic botulinum toxin manufactured by Allergan, removes cosmetic wrinkles from the skin by irreversibly blocking the release of acetylcholine at neuromuscular junctions. In other words, it removes wrinkles because facial muscles are no longer able to contract. Before the companies tested these dangerous drugs in humans, they had to conduct toxicological studies of the drug in animal models. The US Food and Drug Administration doesn't tolerate scientists injecting drugs into humans without a sound rationale.
To do this, scientists choose an appropriate animal model, such as mice, dogs, or other large mammals. Then, they inject the drug at varying dosages via different routes of administration. Following administration, scientists will sacrifice the animal to study the pathology of target tissues. If the target tissues look pink and healthy and not like hamburger, then that's a good support for the drug's safety. If there the drug does indeed cause harmful effects to the animal, then it's up to the toxicologist to determine the cause and circumstances surrounding these effects.
Scientists also want to give enough of the drug to kill the test animals. By finding the dose that kills half of the test animals, the LD50, scientists will have a good rationale a safe dosage range to use in humans in phase I clinical trials.
Even after the drug moves into clinical trials, toxicologists stay on to study the adverse effects of the drug on humans. Many side effects in humans that are unpredictable from an animal model can sink the drug candidate. Nausea is one of them. Mice and dogs can't tell toxicologists that they're suffering from intense nausea, while humans can. Aside from acute adverse effects, toxicologists monitor animals and humans for long term effects also.
Toxicology, along with Pharmacology and Pharmacokinetics are the sciences which companies use in animal testing to provide a rationale for administering a poisonous drug to humans. All of this data is compiled in an Investigational New Drug Application submitted to the US Food and Drug Administration, who then either approves clinical trials, or makes demands for more preclinical data. This intense FDA oversight is what keeps drug R&D costs so high, but is necessary to ensure non-toxic therapeutics for treating disease.