Postmenopausal women on hormone replacement therapy found that those on such therapy had a very small increased risk for Alzheimer's, especially when using combination hormones long-term, a study in Finland found, CNN reports.

The study was published in BMJ.

Women who used hormones for 10 years or more at greater risk

The study  found an additional 9 to 18 per 10,000 women using hormone replacement could be diagnosed each year with Alzheimer's beyond the number expected to develop the disease. The association was highest for women who used hormones for 10 years or more and those who used an oral combination of estrogen-progestogen rather than estrogen alone.

No cause and effect shown

But the study's authors say the results cannot show a cause-and-effect relationship between the use of hormones and an increased risk of Alzheimer's. That's partly because the study was not able to capture age and other risk factors for comparison, such as genetic risk for Alzheimer's, diabetes and cardiovascular disease.

While this large study suggests that women who received some forms of hormone therapy were slightly more likely to be diagnosed with Alzheimer's, it doesn't show that hormone therapy is responsible for this increased risk," said David Reynolds, chief scientific officer of Alzheimer's Research UK, in a statement. Reynolds was not involved in the study.

Conflicting results about the hormone therapy

Numerous studies, including this new research, have presented a stew of conflicting results.

"I am surprised by the results of this study, since other studies have found that HRT actually improves cognitive function," said Channa Jayasena, a clinical senior lecturer at the Imperial College of London, who was also not involved in the study.

Even if HRT increased the future risk of Alzheimer's, several years of treatment would be needed, and the effect is marginal," said Channa Jayasena.  "The results of this study should not change the way HRT is prescribed or viewed," he said in a statement.

Read the whole study HERE