Antibiotic growth promoters are feed additives that are administered at a low, sub therapeutic doses to suppress sensitive populations of bacteria in the gut and improve growth and performance of bird.

According to the National Office of Animal Health (NOAH, 2001), antibiotic growth promoters are used to "help growing animals digest their food more efficiently, get maximum benefit from it and allow them to develop into strong and healthy individuals".

It has been estimated that as much as 6 % of the net energy in the diet could be lost due to microbial fermentation in the intestine (Jensen, 1998) and stimulation of energy-consuming immune responses. If the microbial population could be better controlled, it is possible that the lost energy could be diverted to growth. Whatever the mechanism of action, the result of the use of growth promoters is an improvement in daily growth rates between 1 and 10 % resulting in meat of a better quality, with less fat and increased protein content (Peter Hughes and John Heritage).

Antibiotics prevent thickening of intestine ensuring more nutrient absorption. Thus they spare the critical nutrients for the host by reducing the competition between host and microbes. For example use of AGPs spare glucose by preventing lactic acid production and amino acid by preventing toxic amines production.

The effects of antibiotic growth promoters are much more noticeable in animals housed in crowded, unhygienic conditions. These act essentially in the gut and reduce harmful gram-positive bacteria in the gut, thereby enhancing growth.