Most pharmaceutical products are dosed on the basis of weight (e.g. milligrams of drug per kilogram of body weight). That means an animal twice the size of another gets twice the dose.
Other drugs (mainly chemotherapeutic drugs, like those used for cancer treatment) are dosed based on body surface area (e.g. milligrams of drug per square metre of body surface). With this type of dosing, large individuals get more than small individuals, but the differences are not as great as with weight-based dosing.
Vaccines are a different story. They are administered based on the "antigenic dose" which is independent of body size. Therefore, the same dose is required for an adult Bullmastiff and a young Chihuahua. While it may be tempting to split doses of vaccine between several animals, especially small breeds (and initially this may seem logical (based on their small size) to those who do not realize how the dose is determined), this may result in ineffective vaccination. Trying to save money by splitting vaccine doses can end up costing money through increased risk of disease. Always give the full dose of vaccine as described on the label.
This post originally appeared (in modified form) on www.equIDblog.com on January 4, 2009.