In parallel with pharmacological experiments on efficacy, the toxic effects of acute and chronic dosing are determined. Acute toxicity is less important as long as LD50 (the dose that kills 50% of animals) is not close to the ED50 (the dose causing 50% of maximum pharmacological response).
Chronic toxicity testing is more relevant to clinical applications and should take place along the following lines4: 慢性毒性测试与临床应用关系更为密切，须按下列规则进行：
(1) The route of administration, dose range, dose frequency and plasma levels should be appropriate to likely clinical indications. If possible, methods should be available to measure plasma concentrations and to determine patterns of metabolism.
(2) At least two species should be studied, usually dog and rat or mouse. If possible a species should be selected with a similar profile of metabolism to man.
(3) The duration of treatment should be consistent with the likely duration of use in man and the relative life expectancy of the animal species. Usually toxicity studies are undertaken over a period of 4 weeks to at least 1 year.
(4) Hematological and biochemical measurements should be made serially. All tissues should be examined histologically at death or on sacrifice of the experimental group. An untreated control group of littermates should be maintained for comparison.
Depending on the proposed patient group and disease indication, attention must be paid to:
(a) Effects on fertility in both males and females.
(b) Teratogenic effects on development of the embryo. The vulnerable period is very early in development, during organogenesis.
(c) Mutagenicity or an increased rate of mutation in germ cell lines or non-reproductive cells, e.g. bone marrow.
(d) Carcinogenicity or the induction or promotion of malignant tumors.
There is disagreement over the relevance of some animal carcinogenicity studies to man5.
Extensive formal toxicological tests are now required in most countries before drugs can be used on patients. There is considerable controversy as to the value of routine toxicology testing, as many differences between species, especially between man and rat, mouse, and dog, have been reported6. 对于有关一些动物致癌性研究与人类的相关问题，尚存有分歧。
Paradoxically, thalidomide, which was the cause of the tragedy that led to stricter drug regulation and toxicology tests, is not teratogenic in mice or rats but has a teratogenic effect in humans, causing gross limb deformities.