Mild protocols are those that at worst give rise to slight or transitory minor adverse effects. Substantial protocols are those that may result in a major departure from the animal's usual state of health or well being. Moderate protocols can best be defined as those that fall between mild and substantial.
Severity limits cannot be determined by a single adverse effect. Professional judgment is required to consider and weigh all the expected adverse effects before assessing the appropriate severity limit. The duration and intensity of the suffering are always taken into account. Further details of the criteria used to assign severity limits to regulated procedures are set out in paragraphs 5.40 to 5.49 of the Guidance on the Operation of the Animals (Scientific Procedures) Act 1986 (HC321). Copies of the guidance have been placed in the House Library.