Looking after somebody with dementia can be very taxing
More than half of those looking after a relative with dementia told researchers that they had mistreated them.
The University College London research revealed that a third admitted "significant abuse".
Verbal abuse or threats were common, but just three of the 220 people questioned in the British Medical Journal study admitted physical abuse.
The Alzheimer's Society described the findings as "shocking", but said that many carers were under great strain.
The UCL researchers interviewed people caring for relatives with dementia in their own homes.
The patients involved had just been referred to hospital psychiatric services for their condition.
In total 115 carers reported at least some abusive behaviour, and 74 reported more serious levels of mistreatment.
More than a quarter of the carers admitted screaming or yelling at their relative, while just under one in five said they had used a harsh tone or had sworn.
Other abuse recorded included threats to send the relative to a care home, or to stop caring for them.
A far smaller number admitted hitting, slapping, shaking or rough handling of the person with dementia.
'All or nothing'
The researchers, led by Dr Claudia Cooper, said that professionals tended to avoid the issue when talking to relatives.
They wrote: "Professionals are often reluctant to talk about abuse, perhaps because of a fear that discussing and acknowledging it would necessitate referral of an adult for protection and trigger a punitive response such as removal of the person with dementia.
"This may result in an 'all or nothing' approach to abuse, where it is ignored until the problem becomes serious.
"Similarly clinicians may not consider abuse when seeing most carers, if abuse if perceived as a rare action purposefully perpetrated by amoral abusers."
However, the Alzheimer's Society said that the abuse of people with dementia should be considered in the same way as child abuse.
A spokesman said: "People with dementia are the most vulnerable in society and it is shocking that this study has found that they are being subjected to abuse in their own home.
"We need to ask why it is happening - most carers do an excellent job in very difficult circumstances, but without help and support they are placed under enormous strain.
"Giving carers access to respite, psychological support and financial security could help end mistreatment."
The government is currently consulting on a new policy for the safeguarding of "vulnerable adults", but the researchers said this was primarily focused on preventing abuse by paid carers, rather than family members, and called for it to be extended.
Rebecca Wood, chief executive of the Alzheimer's Research Trust, said the findings were "unsettling".
"Dementia requires a particularly dedicated form of care that puts enormous human and economic strains on our society.
"The UK is facing a dementia crisis - the number of people with the condition will hit 1.5m within a generation.
"The government must do much more to support carers, protect patients and fund more research into treatments for this terrible disease."