About 65,000 people in Scotland have dementia and about 40% are in care homes or hospitals.
The Care Commission and Mental Welfare Commission visited a sample of homes between August and March and detailed their findings in a report, Remember I'm Still Me.
It said before the patients were admitted most had already had a good medical assessment, with input from GPs and dieticians.
But after admission, the report said very few had annual GP health checks and there was little evidence that medication was regularly reviewed.
The organisations expressed concern that medication was being misused, with some residents on a cocktail of antipsychotics, and some being prescribed drugs known to be dangerous.
In nine homes, the watchdogs said medication was being illegally disguised in food or drink, probably to make residents easier to manage.
About half of the residents never went out of the homes, the report said, and only a tiny minority could come and go even into secure internal gardens.
The commissions also looked at 182 personal plans to see how people's life histories were recorded and found that less than a quarter had adequate details.
Just over a third of the personal plans recorded no life history at all.