More people than previously thought could be at higher risk of having a stroke caused by their antipsychotic drugs, say UK scientists.
Previous research suggested only some types of the drug increased the risk, particularly for people with dementia.
However a study published in the British Medical Journal says all forms of antipsychotics boost the risk, in all patients.
A mental health charity said patients on the drugs must be closely monitored.
Antipsychotic drugs are generally used to control psychotic symptoms in patients with disorders such as schizophrenia, and some severe forms of depression.
They are also thought to be widely used to control symptoms of dementia such as aggression, leading to accusations they were being used unnecessarily as a "chemical cosh" in some circumstances.
They fall into two types - newer "atypical" and older "typical" antipsychotics.
When the first concerns were raised in 2002, these focused on the "atypical" drugs.
These worries led to a recommendation from drug safety watchdogs in the UK that they not be given to people with dementia, and the government has been urged to strengthen this in England in its forthcoming dementia strategy.
The latest findings, from researchers at the London School of Hygiene and Tropical Medicine, confirm the fears over dementia patients, but raise wider concerns.
They identified 6,700 patients from a GP database, all with an average age of 80, and concluded that there was more than a tripling of risk for dementia patients taking any sort of anti-psychotic drug.
Patients without dementia taking any sort of antipsychotic had a 40% increase in risk.
The researchers repeated the recommendation that patients with dementia should not be prescribed these drugs.
Neil Hunt, from the Alzheimer's Society, said that doctors now needed to heed these warnings.
"The over-prescription of antipsychotics is a serious breach of human rights, these drugs should only be a last resort.
"The forthcoming National Dementia Strategy is a crucial opportunity to stop this dangerous over-prescribing and we look forward to its launch in the autumn."
Marjorie Wallace, the chief executive of the mental health charity Sane, said that while the drugs were capable of transforming lives, different patients reacted differently to their side-effects.
"This study should remind us all that antipsychotics are powerful drugs which can both be essential for some people, while carrying other risks.
"This is another warning that all antipsychotics should be prescribed with great thought and care and be subject to rigorous follow-up."