Like humans, domestic animals require medications when they fall ill. Since weight differences in animals can be as large as 700-fold, accurate weight information is necessary in determining the dosage needed by each specific animal. Like humans, domestic animals require medications when they fall ill. Since weight differences in animals can be as large as 700-fold, accurate weight information is necessary in determining the dosage needed by each specific animal. Weight is an important factor that affects drug absorption and distribution throughout body tissues. Due to variations in weight among animals, many drug dosages are adjusted according to the weight of each individual animal. These drugs are generally dosed on a milligram per kilogram (mg/kg) basis. That is, if an animal weighs 10 kg and the drug to be administered is dosed as 4 mg/kg, then the animal will receive a total of 40 mg of drug. Veterinary scales used for weighing animals should have a resolution of at least 0.05 kg, or 0.1 lb. Administering the proper dose is necessary to maximize the desired therapeutic effect while minimizing adverse effects. Some drugs have weaker dose-response relationships and allow for a greater margin of error. Other drugs, such as chemotherapy drugs used for treating cancer and anti-seizure drugs, are considered narrow therapeutic index drugs. The therapeutic index is said to be Narrow because there is a fine line between the toxic dose and the therapeutic dose. Since there is little room for error with these types of drugs, proper dosing is crucial because a slight miscalculation could lead to serious adverse effects, or even death. Antibiotics are among the most commonly prescribed drugs in veterinary practice, and are another type of drug in which accurate dosing is important, although for a different reason. When administering antibiotics, the greatest concern is that the bacteria will become resistant to the drug. This is often the case when the initial dose of the antibiotic is too low and/or the length of treatment is too long, leading to bacterial resistance, which can render subsequent doses useless even when done correctly. Too high a dose, on the other hand, can lead to toxic side effects. When starting any drug therapy, treatment should be initiated at a low dose to decrease side effects. Any changes and improvements in the animal’s condition should be monitored regularly. If there is no improvement in the animal’s condition, it would then be necessary to gradually increase the dose at regular intervals until the therapeutic effects are maximal. Throughout the dose adjustment process, side effects should remain at a level that is tolerable for the animal. An important variable for achieving therapeutic effect is bioavailability, which is dependent on drug concentrations in blood plasma. Weight and body surface area determine blood volume, which in turn determines plasma drug concentrations. The binding of drug to albumin proteins in the blood can also decrease bioavailability, since only free, unbound drugs can have effects on the body. Another weight-related factor in determining drug dosages applies mostly to fat-soluble drugs. Fat-soluble drugs such as barbiturates and thiopental are absorbed into fat tissues and may be present in the body for as long as three hours after administration. Therefore, these drugs would require less frequent dosing, at lower doses. Animals are treated for diseases such infections, heartworms, fleas and ticks, and diabetes. The correct medication dosage is necessary for improving the animal’s health without doing any harm. A balance must be achieved between the toxic and beneficial effects of a drug. The veterinarian must be aware of the different factors that affect drug dosages, weight being an especially important one.