While people tend to think sedatives & tranquilizers are the same thing, they are different and have different effects on our dogs. In addition, no 2 drugs are the same within each definition. I wanted to share some information regarding a popular tranquilizer called Acepromazine, or "Ace". Ace is used incredibly frequently in our animals during any procedures where they are "put under", but it is often prescribed for use at home. A lot of owners give their dogs Ace to calm them during thunderstorms, moving, etc. so I figured it would be of interest for some to read more about it since it is a commonly used drug in veterinary medicine.
The largest difference between tranquilizers and sedatives, is that while they both produce a calming effect, sedatives also offer some analgesia (pain relief). A book of mine also suggests that while on sedatives, your dog isn't aware of it's surroundings, while on a tranquilizer they are aware, but don't care. A sedative with an analgesic effect is a great option for use before a surgery, so they have some proactive pain relief on board. But your vet may also use a tranquilizer before a procedure, to merely calm your animal down, and additional analgesics are used.
Ace is very effective in producing a calming effect, and it decreases motor activity. It lasts about 4-8 hours, but in older dogs or ones with liver disease, effects can last up to 2 days. This may be a reason why your vet doesn't want to give your dog Ace if they are older. You may think 'something went wrong' and not realize that this drug, and some others, take a while to run their course.
Ace can cause your dog's thermoregulation center to get a little screwy, resulting in lowered body temperatures. It also has a slight anti-emetic (vomiting) effect. Contrary to what you may think, a drug like this may actually elevate your dog's heart rate. We think "tranquilizer", but in reality this drug may slow your dog's blood pressure. It's body will try to compensate by raising the heart rate. Always ask your vet what to expect, and what would be considered normal for your dog specifically. This drug can also cause your dog's third eyelid (nictitating membrane) to pull over his eyes, but as a side-effect, not as a permanent condition.
The most interesting side effect I read about Ace, is personality changes. One of my books suggested your dog may even bite. It's important to understand this is a potential side effect. I could imagine an owner bringing their dog home after a procedure where Ace was used, and thought the dog must have had a bad experience emotionally, since now they are behaving negatively. I imagine if a dog snipped at it's owner, they'd apply human emotions and assume their dog was mad, or that it hated the vet. Or worse, something 'bad' happened to him while he was there.
I had a friend use a tranquilizer on her cat during plane travel. I don't recall which kind. Her theory was that it didn't appear to be working, so she gave her cat more until she thought she reached a desired effect. The thing that is crucial to remember, is that "more is not better". Your recommended dose will achieve the best effect possible, and more unwarranted amounts may only serve to increase the amount of negative side effects. Remember all drugs have negative side effects, and some may even seem to go against the drug itself. Cats, for instance, can tend to have strange reactions to some drugs.. even becoming more alert while they should be more calm.