The Tower Bridge Care Home at the Bricklayers Arms has been criticised by the Care Quality Commission in its latest report.
A year ago the care home was rated 'good' by the CQC after several years of poor ratings.
However, the latest CQC inspection raised a number of concerns and the home now has a rating of 'requires improvement'.
100 people live at the four-storey care home overlooking the Bricklayers Arms roundabout.
Concerns raised by inspectors included problems with the administration of medicine; they found that one resident had missed some of their painkillers for around 10 days because of a paperwork error.
Inspectors found that the service was not always meeting the requirements of the Mental Capacity Act 2005 (MCA). "We saw one example of a person's liberty being restricted without evidence that the decision was made in their best interest," said the CQC.
The report also warned that "the provider did not always ensure persons employed for the purposes of carrying on a regulated activity were of good character".
A spokswoman for HC-One, owners of the care home, said: "The health, safety and wellbeing of residents is our number one priority. We are pleased with the positive feedback we received from the CQC regarding the responsiveness of care at the home, but we were disappointed with the areas highlighted as requiring improvement.
"We take all feedback from the regulator very seriously. A robust action plan is in place at the home and we are working closely with both the CQC and the local authority to address the concerns raised.
"Since the inspection we have taken steps to address the points raised by the CQC, including: implementing a strengthened system of weekly medications checks; reviewing and updating care plans for all residents; improving medication management training for staff; and completing a full audit of personnel files for staff at the home.
"We are pleased with the progress we are making, and we look forward to welcoming the CQC back to the home in the near future."