In some cases, a veterinarian may prescribe drugs to treat serious separation anxiety. Reconcile, for example, is one such drug that’s new on the market; it’s based on Prozac.
Desensitization is the method most often used to treat separation anxiety, however. It entails gradually acclimating your dog to your departure. It is strongly recommended that you seek help from a reputable behaviorist if you think desensitizationis your best treatment option. It usually takes eight weeks or less to bring symptoms under control; in rare cases, much longer.
For now, here’s a very simplified example of how you would approach the problem: The first step is to get your dog used to hearing the sound of your keys jingling. When he can do that without exhibiting any signs of distress, add picking up your briefcase. Then add walking to the door. Then opening the door. You’ll continue adding actions, in baby steps, until you can leave the house for a period of an hour or more without consequence.
If this seems like a slow and tedious process, it is. In the meantime, keep your greetings and goodbyes as low-key as possible. This signals to your dog that coming and going are casual, common occurrences–no need for drama or spectacular displays of emotion.