Aloha! You're speaking with Dr. Michael Salkin
To answer you directly, yes, a benzodiazepine such as diazepam can be valuable in dogs such as Harry. I dose diazepam at 0.5-2 mg/kg preferably 30 minutes to one hour in advance of its necessity. Ultimately, however, the etiology of his howling needs to be addressed and separation anxiety is the most likely etiology. Please peruse my synopsis of this anxiety below. Not everything you read will be applicable to your situation but it's a good start in understanding why Harry is behaving in this manner.
When a dog has a very close relationship with a family member, it may become anxious when it suddenly loses access to that person. Anxiety-based problems, including separation anxiety, tend to occur with increased frequency and intensity in the adult/older pet dog population. Situations such as changes in the family members' work schedule or returning to work after a long stay at home can lead to this type of problem. The dog with a separation anxiety problem may show signs of either increased activity and anxiety (pacing, restlessness, whining) or depression (lying around, reluctance to move or eat) as the owner prepares to leave. These behaviors often begin as the pet becomes aware of predeparture cues, such as putting on a coat, reaching for keys, or picking up a briefcase. When the owner returns home, the dog usually exhibits high levels or arousal and may show exaggerated greeting behaviors. Separation anxiety can also occur when the owner becomes involved in an activity or relationship that takes a significant amount of attention away from the pet at home. The anxiety then becomes a driving force for excessive vocalization, self-mutilation, destructive behavior, or housesoiling. An important clue that differentiates separation anxiety-related housesoiling from other causes is that these pets will often eliminate in the home every time the owner leaves, even when the absence is of very short duration, and in spite of the fact that the pet has eliminated in an appropriate area just prior to the owner's departure.
Treatment involves desensitization to predeparture cues and gradually accustoming the dog to separation from the owner. If the owner can provide a dramatic increase in daily exercise, this will usually have a calming effect. Enriching the pet's environment (rubber toys stuffed with treats) or distractions (another pet, radio) may help, although some dogs experience such high anxiety that food and distractions are ignored. During the early stages of treatment, a small confinement area, a pet sitter, or boarding may be necessary and general principles of housetraining should be followed. Drug therapy with tricyclic antidepressants (clomipramine, amitriptyline) and benzodiazepines (diazepam, clorazepate, alprazolam) may be helpful when the anxiety is intense.
Please see my essay below for a more formal approach to separation anxiety and respond with further questions or concerns if you wish.
The discussion of separation anxiety is exhaustive - lectures I've attended encompass hours on this problematic behavior alone. I can, however, give you a synopsis of its management that should be helpful. If possible it would be best if you could have a board certified animal behaviorist come to your home to better evaluate the dynamics that exist there. His vet should be able to refer you to such a specialist or you can find one here: www.dacvb.com
Step 1) Change the relationship - Teach Harry independence. He should not be allowed to get attention on demand. When he gets what he wants every time he nudges or whines, he is more likely to be anxious when he is alone and can't get social attention. You can give him attention when you so desire but it must always be on your terms, not Harry's.
Step 2) Departures and predeparture cues - Departures should be kept as calm as possible. The presence of certain departure cues will typically create anxiety about an impending absence of you. Harry should be desensitized to those cues that can't be avoided during departure. You should repeatedly pick up the car keys, open, shut, and handle the door, put on a coat or pick up a briefcase so that he habituates to these cues and they lose their strength in eliciting anxiety. Placing him in his cage, locking it in the kitchen, or opening and shutting the door are events that Harry should be constantly exposed to when you're at home, during sit-stay and reward training sessions. Until he has been desensitized to these cues, they should be avoided whenever possible during actual departures. Putting jacket and boots on in a different room, leaving a briefcase, handbag or key in the garage, and leaving through a different door while he is otherwise occupied or distracted can greatly help reduce departure anxiety. Cues that are commonly associated with calmness, food, and your presence can be provided during departure to reduce anxiety. During departures, a TV, radio, or videotape can be left on, or Harry can be provided with a favorite blanket to lie on. Some owners do not understand the principles of these techniques so that the dog is placed in a cage or a radio turned on only when the owner leaves, so that these cues become associated with anxiety and departure, not calmness.
Step 3) Greetings - Homecoming should be kept very low key and Harry should be ignored until he is calm.
Step 4) Obedience - Teach "sit", "down" and "stay" commands so you can begin teaching him to tolerate being alone.
Step 5) Teach the pet to be alone - phase 1 - This phase should begin with Harry staying for a very short period before accompanying you to various rooms throughout the home. Gradually, he should be required to stay for longer periods of time, until he will remain in another room for 30-60 minutes or more.
Step 6) Teach the pet to be alone - phase 2 - After Harry has been desensitized to the departure cues, you should practice short mock departures. You should initially leave for a very short period of only a few seconds to a few minutes. The duration should be shorter than the time in which it takes him to show signs of anxiety. Periods can be lengthened gradually as Harry responds without associated anxiety. The duration of departure should be lengthened on a variable schedule, so that he can't predict exactly how long you'll be gone.
Step 7) Exercise - Lots of aerobic exercise should be provided.
Step 8) Distractions - Harry may be less anxious when he has something to do while left alone. highly stimulating toys should be provided. New chew toys, food chews (pigs' ears, rawhide) or strongly motivating food pieces hidden in the toys, such as meat or cheese may get his interest. These treats can be hidden inside toys so that they are difficult to remove, in packages that he must open, or hidden under bowls around the home. In rare situations, having another pet will provide a playmate (or distraction) for a dog.
Step 9) Confinement - May result in increased anxiety unless he is already accustomed to confinement. Acclimating a pet to confinement should be done gradually. If this is not practical, anxiolytic medication (benzodiazepines, TCAs, SSRIs, buspirone) or D.A.P./Adaptil may be useful.
Step 10) Punishment - Punishment should be avoided as should any other treatment modality that might cause anxiety.
Step 11) Hormones/Drugs - Dog Appeasing Hormone (D.A.P./Adaptil available online) may reduce anxiety, especially in primary hyperattachment disorders. Tricyclic antidepressants such as clomipramine (Clomicalm from his vet) are a good choice for chronic anxiety problems and have proven efficacy in clinical trials. Fluoxetine or other SSRIs may be an effective alternative. Benzodiazepines such as diazepam, alprazolam and clorazepate may also be useful for immediate control of severely affected pets, especially those that have panic attacks. Other drugs such as barbiturates, propranolol, buspirone, and phenothiazines may also be helpful adjuncts to behavioral therapy techniques. However, on their own, they are rarely successful for treating severely affected pets.
As you can imagine, this behavior is difficult to address. I'm hopeful with your understanding this behavior better and by following the guidelines presented above you'll help him address his anxiety. We should see positive results within 6 weeks. The prognosis is poor for dogs who don't respond within that time frame. Please respond with further questions or concerns if you wish.