Care homes are being warned that they may be breaking the law and putting patients' lives at risk by hiding drugs in food or drink, the BBC has learned.
Research suggests the practice takes place in 70% of homes
The Mental Welfare Commission is concerned that the practice of giving medicine secretly is being over-used.
Under Scots law, medical staff can intervene in a patient's treatment if the person is incapable of acting in his or her own best interests.
But the law is not clear on whether this includes concealing drugs.
Research has suggested that the practice takes place in 70% of care homes.
The Mental Welfare Commission, which exists to protect the rights of patients, is concerned that what should be a last resort is actually becoming normal practice.
It has issued new guidance, warning staff that they risk breaking the law if they have not explored other options and that crushing medicines into food can make them dangerous.
Aberdeen resident, Hunter Watson, saw staff in care homes administer the secret medication.
When his mother was moved into a care home at nearly 90, she was given drugs without her knowledge.
Mr Watson said: "Shortly after entering the home I visited her and found her unable to get out of bed. She thought she had been given something and when I enquired she had had an anti-psychotic drug concealed in her sweet."
Mr Watson complained and the secret medication was stopped, until the care home changed ownership.
He explained: "I saw a care assistant squirting something into her juice with a syringe.
"I found out later from the doctor that the staff had again requested that my mother be sedated. The doctor didn't think it necessary but acceded the request and this was to be done to my mother for life."
Dr Donald Lyons is the chief executive of the Mental Welfare Commission.
He said: "About 70% of care homes for people with dementia have used covert medication at some point and we are worried that because this practice goes on, and goes on in secret, then people might not be following best practice.
"It is not that it should never be given, but it has to be given for the right reasons, that is for someone's benefit.
"You must always make sure if you are giving it, it has to be given safely because if it means crushing tablets or combining medication with food then it might not be safe, and it could actually be quite dangerous for people."
Medication is sometimes crushed or combined with food
In 2002 the issue came up in the Royal College of Nursing's annual congress.
The gathering heard that drugs were too often hidden for the convenience of staff rather than in the best interest of patients.
But Alan Jacques, a retired psychiatrist who now works with Alzheimer Scotland, said it was often done with the backing of relatives worried about their loved ones.
He said: "This is something that has to happen from time to time, I saw it happening in places around the country, it has been very common practice.
"Our concern is not that it is happening but that it is happening without any proper control or consideration of good procedures."
In Scotland, the Adults with Incapacity Act of 2000 and the Mental Health Act of 2003 allows staff to give medical treatment to those who are incapable of deciding for themselves, but it forbids treatment by force.
Mr Watson's advice to care home staff was "don't do it".
He added: "I think people should go into care homes and enjoy their food and drink without worrying what these might conceal.
"And if staff think a resident should have an anti-psychotic drug then they should apply for powers under the 2003 Mental Health Act."