More than half of carers looking after a relative with dementia have been abusive towards them, experts said.
While most abuse seems to be verbal, such as shouting, some carers physically assault the person in their care.
Experts from University College London (UCL) questioned 220 carers looking after people with dementia at home. Of these, 115 (52%) had been abusive in some way to the person in their care, with 74 (34%) reporting significant levels of abuse such as swearing often or being insulting.
Verbal abuse was the most commonly reported, according to the study in the British Medical Journal (BMJ). Three (1.4%) carers said they occasionally physically abused the person in their care.
Dr Claudia Cooper, from the UCL department of mental health sciences, who led the study, said: "Many people think about elder abuse in terms of 'lashing out' and other similar acts, but abuse as defined by Government guidelines can be as simple as shouting or swearing at the person being cared for.
"This is the first representative survey to ask family carers about abuse. It shows that abusive behaviour towards people with dementia from family carers is common according to the scale used, with a third reporting 'significant' levels of abuse, and half some abusive behaviour.
"We found few cases of physical or frequent abuse, although those with the most abusive behaviour may have been reluctant to report it, or take part in the study in the first place."
The researchers used a scale to measure abuse called the Modified Conflict Tactics Scale (MCTS). Carers answered questions about how often in the last three months they had acted in five psychologically and five physically abusive ways on a scale of 0-4 (1 = never, 4 = all the time). A score of more then 2 on one question is defined as "significant" abuse.
Co-author Professor Gill Livingston said: "Our findings suggest that any strategy for safeguarding vulnerable adults must be directed towards families who provide the majority of care for older people, rather than exclusively at paid carers."
There are 700,000 people living with dementia in the UK.
