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Ask the Vet: Drug Dosages

Marcia Rybak and Stephen L. Barten, D.V.M.

As the popularity of keeping reptiles grows so do their medical problems. Traditionally, cures were administered only by veterinarians who specialized in treating reptiles and other exotic animals. Today, with herpetologists maintaining larger collections, a type of "herd health" is often practiced, where the veterinarian provides medicines that you can take home and administer to the entire collection by yourself. Home treatment presents its own problems and concerns, one of which is calculating the correct dosage of the oral or injectable medicines to be given to each of your reptiles. Too little and the medicine is ineffective. Too much and your reptile could suffer an overdose which might cure the disease but kill the patient.

Drug doses were initially calculated by extrapolating established doses used for mammals and applying them to reptiles based on their weight. Recently, as a result of the experience we have gained, there are now standardized doses for many drugs used on reptiles. Some are based on pharmacokinetic studies, where a drug is given at various doses to a group of reptiles and blood samples are taken at regular intervals afterwards to check the level of drug in the bloodstream. This allows both the dose and the interval between doses to be calculated scientifically. More pharmacokinetic studies on reptiles are being published all the time, but few exist compared to the number of drugs available. For example, no pharmacokinetic studies on any lizard have been published, and their doses are usually extrapolated from those of snakes. The other basis for drug doses is empirical. Put simply, this is the dose that seems to produce the desired effect without causing any detectable side effects. Empirical doses are based on the experiences of others, and are far from scientific, but also are better than nothing. Recommended drug dosages for reptiles are available from your veterinarian or can be found in published materials such as those listed in the references.

A dose is the amount of medicine to be administered at on time. It is expressed in terms of milligrams of medicine to be giver per kilogram of body weight of the animal. With this information there are still four variables that you must know when medicating your animal. These are the accurate weight of each patient converted to kilograms, the concentration of medicine being used, the interval between doses, and the duration of treatment. Although this sounds complicated, there is a simple formula you can use to arrive at the exact single dosage for the reptile you want to treat.

The first step is to convert the weight of your reptile to kilograms. For small reptiles you will probably measure the weight in grams or ounces. Convert grams to kilograms by dividing by 1000 or moving the decimal 3 places to the left. A ball python weighing 876 grams weighs 0.876 kilograms. There are 28 grams in an ounce, so to convert ounces to kilograms, multiply the ounces by 0.028. For bigger reptiles, a pound equals 0.454 kilograms, so multiply pounds by 0.454 to get kilograms.

1000 grams = 1 kilogram
1 pound = 0.454 kilograms
28 grams = 1 ounce
2.2 pounds = 1 kilogram
454 grams = 1 pound

In the past it was common practice to subtract half of the body weight when calculating a dose for a turtle to account for the weight of the shell. This should not be done because the shell is living tissue. Doses for turtles should be calculated based on total body weight.

The second step is to understand how strong the medicine is that you are using. This is known as the concentration and is usually expressed in mg/ml (milligrams per milliliter) and should be clearly written on the bottle. Occasionally the concentration may be stated in mg/cc (milligrams per cubic centimeter), but ml and cc are different names for the same unit of measurement. This is important to know because while most bottles are marked in ml, all syringes used to administer the actual dose are marked in cc. If a concentration is stated in per cent, multiply the per cent by 10 to get the concentration in mg/ml. For instance, Baytril comes a s 2.27% solution. Multiply 2.27 by 10, and there are 22.7 mg/ml.

To arrive at the correct dosage, multiply the dose times the weight of the reptile and divide by the concentration. Here' an example. Suppose the ball python mentioned above has parasites and needs to be treated with fenbendazole which is concentrated at 100 mg/ml. The prescribed dose for ball pythons is 25 mg/kg administered orally once and repeated 3 times at 2 week intervals or until a negative fecal is obtained. The formula to calculate the dosage works as follows:

dose x weight 25 mg/kg x 0.876 kg
------------------- = --------------------------- = 0.219 ml
concentration 100 mg/ml

In this case you would round 0.219 cc up to 0.22 cc and administer the dose. It is crucial that you convert dose, weight, and concentration to these units of measurement. If you multiply a dose in mg/kg by a weight in grams or ounces, your calculated dose will be wrong and you might overdose or underdose the patient. The same holds true if you divide the dose x weight by a concentration in any units other than mg/ml. Always double check your calculations.

The frequency and duration of the medication can be found in the references or can be obtained from your veterinarian.

There is another way to calculate dosage which is based on the animal's size and the knowledge that a smaller animal has a higher metabolism than a larger one. Animals are divided into metabolic groups and a scaling constant is applied to the formula. The method is called "metabolic scaling" and uses more complicated ratios than are shown here. This method is discussed in the references.

I would caution you that when doubt, check with your veterinarian regarding the treatment program you are considering using for a reptile with which he is not familiar. Antibiotics and parasiticides are potent medicines and should only be used when necessary.

Frye, F. L., 1991. Biomedical and Surgical Aspects of Captive Reptile Husbandry, second ed. Krieger Publishing, Inc., Melbourne, FL. (The same book is also published as Frye, F.L., Reptile Care: an Atlas of Diseases and Treatments. TFH Publications, Inc., Neptune City, N.J., 1991)

Mader D.R.: Antibiotic Therapy. In Frye, F.L.: Biomedical and Surgical Aspects of Captive Reptile Husbandry, second ed. Malabar, FL, Krieger Publishing, Inc., 1991, pp. 661-634

Pokras, M.A., C.J. Sedgwick, and G.E. Kaufman. Therapeutics. In P.H. Beynon, M.P.C. Lawton, and J.E. Cooper, eds. Manual of Reptiles. British Small Animal Veterinary Association, Gloucestershire, England, 1992, pp. 194-213