Is there an effective medication or other treatment for dementia?

No. The medication used to treat dementia slow the progress but do not reverse the process.
More or less. Actual treatments for dementia -alzheimer's type do not cure but may delay the progress of it. Among those...Aricept (donazepil) namenda, galantamine, (galantamine hydrobromide) exelon, axona. The only treatable dementias are caused by syphilis, B12 deficit, thyroid disease. Hydrocephalia causing dementia requires a surgical shunt to eliminate the CNS fluid.
Depends. There are many different types of dementia. Many symptoms can be treated but this does not usually change the underlying course of the illness.