Some commonly prescribed statins can impair memory but others do not, scientists have found.
Between six and seven million people in the UK take the medicines every day to lower "bad cholesterol" in the blood.
But after starting the treatment, some patients complain that their memory is affected.
Last year, the US Food and Drug Administration (FDA) insisted all manufacturers list in their side effects that statins could affect cognitive function.
Scientists at the University of Bristol tested the effects of two commonly prescribed statins - pravastatin and atorvastatin - on rats.
Pravastatin, with the brand name Pravachol, was found to have adverse effects on working and recognition memory.
However, atorvastatin, with the brand name Lipitor, did not have any effect.
The study, published in PLOS ONE, found adverse effects of pravastatin on memory could be reversed by stopping the medication.
Neil Marrion, professor of neuroscience at Bristol's School of Physiology and Pharmacology and the study's lead author, said: "This finding is novel and likely reflects both the anecdotal reports and FDA advice.
"What is most interesting is that it is not a feature of all statins.
"However, in order to better understand the relationship between statin treatment and cognitive function, further studies are needed."
The research examined adverse effects on memory from prescribed statin medicines, used to lower low-density lipoprotein (LDL) in the blood.
Results showed rat performance for simple learning and memory tasks were impaired when taking pravastatin, but not atorvastatin.
The rats were treated daily with pravastatin or atorvastatin for 18 days.
The rodents were tested in a simple learning task before, during and after treatment, in which they had to learn where to find a food reward.
The rats also performed a task which measured their ability to recognise a previously encountered object, on the last day of treatment and a week after it finished.
Pravastatin tended to impair learning over the last few days of treatment, though this was fully reversed once the rats stopped taking the medicine.
In the novel object discrimination task, object recognition memory was also impaired by pravastatin.
No effects were observed for atorvastatin in either task.
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