Not only can they have dangerous side affects, but they can cause imminent death when combined either together or with other drugs. Basically any drug used on the older pet is just buying the dog a little more time and trying to ease the quality of life. But by the same token, you don't want to destroy them doing it. There are orthopedic beds with lots of foam rubber, made especially with the aging pet in mind. Also, heated pads or bedding might help to make the aging pet more comfortable.
Many Vets recommend occasional health checks and blood work as a form of preventative medicine.
Blood work especially can give early warning to pending health problems. This is especially a good idea if your aging dog just doesn't seem right. Anemia can often be a problem in the aging dog, as well as lower Thyroid levels. It is good to keep on top of these health issues, before something gets out of control. Situations that a younger dog brushes off, can become a calamity for the oldster.