Wiltshire Heights, in Bradford-on-Avon, was rated as "inadequate" in the.
Inspectors found found that staff were not administering medicines safely, and the home had no registered manager.
Porthaven Care Homes Ltd, which runs Wiltshire Heights, was approached by the BBC for a comment.
The home provides care for 63 adults, some of whom suffer from dementia or exhibit "challenging" behaviour.
The report found that the home was "inadequate" in four of its five indicators, and "required improvement" in the fifth.
In one instance, a duty nurse told inspectors that a patient had run out of medicine because the staff member who usually ordered it was on holiday.
Staff were also found to be inadequately trained in taking patients' blood or handling a syringe.
Inspectors also found that staff were not trained to manage residents with challenging behaviours, and as a consequence their safety and the safety of other residents was compromised.
The report was compiled following an unannounced inspection which took place across two days in August.