I frequently write prescriptions for my patients, especially for medications I do not keep in stock, however, many of my clients prefer the convenience of our in-house pharmacy. A far more important consideration is whether an outside pharmacy is comfortable with non-human patients. Variables such as dosage and dosage interval are frequently very different than they are in humans, and I don't want pharmacists trained in human medicine altering my prescription (without calling) because they believe I've made a mistake. Client education is also a concern. It's our responsibility as veterinarians to inform the client about usage and potential side effects of any medication we prescribe. That said, it's much easier to do so effectively when the medication is available in-house, and you can demo dosing and administration for your client. While pharmacists often counsel their clients on their medications, they are ill-equipped to provide the same service for non-human patients.