(CNN) Some of the drugs most commonly used to treat people with high blood pressure could also be affecting their mood -- particularly their risk of depression -- according to a new new study .

Research published in the journal Hypertension found that people taking one of two classes of drugs, known beta blockers or calcium channel antagonists, had twice the risk of being admitted into the hospital with a mood disorder, such as severe depression. However, people taking a class of drugs known as angiotensin blockers -- ACE inhibitors -- had a lower risk of developing severe mood disorders, even compared with healthy control groups with no history of hypertension or depression.

"They are both extremely important findings," said Dr. Sandosh Padmanabhan, professor of cardiovascular genomics and therapeutics at the University of Glasgow and lead author of the study. "And they were both unexpected."

Patients taking a fourth class of drugs, known as diuretics, were found to have the same risk of developing mood disorders as people not taking any form of treatment.

"There is a lot of data that depression and cardiovascular disease are related ... but current hypertensive practices do not consider depression," said Padmanabhan. "This validated those (earlier) findings, but also means blood pressure tablets could be repurposed for mental health conditions."

High blood pressure is estimated to cause 7.5 million deaths worldwide each year, according to the World Health Organization . Almost a billion people around the world lived with uncontrolled hypertension -- high blood pressure -- in 2008, and that number is expected to rise to more than 1.5 billion by 2025. As more people develop hypertension, it's expected that more will be taking these blood pressure-lowering drugs.

Medication and your mind

Previous studies have showed links between anti-hypertensive drugs and mental health. One small study in the United States found that calcium channel blockers helped improve symptoms of bipolar disorder, while drugs such as beta blockers have been linked to depression for some time.

"We know that some hypertensive drugs cause depressive symptoms," Padmanabhan said, citing beta blockers and another class known as methyldopa. These two classes of drugs are more often prescribed during pregnancy, as they are not harmful to fetuses, but the researchers would now like to investigate whether they could play a role in postpartum depression.

"Nobody has studied this," he said.

Exposing the link

The researchers collected data from a database of more than 520,000 patients across two hospitals in Scotland. From this, they selected 144,066 patients who were not diagnosed with depression but were being treated for hypertension exclusively with one of the four chosen classes of drugs to be included in the study: beta blockers, calcium channel blockers, angiotensin blockers or diuretics.

These subjects were compared with more than 111,000 people who were not receiving any form of treatment for either hypertension or depression, over a five-year period. The numbers admitted to a hospital for severe mood disorders, such as depression of bipolar disorder, were monitored.

"As expected, beta blockers had a higher rate of admission for major depression," Padmanabhan said. The same was true for calcium channel blockers. "(But) the angiotensin group had lower rates than the control group. That was surprising."

The researchers warn that the study has many caveats, notably the fact that the findings are purely observational and monitored only for major mood disorders, so minor changes or minor depressive episodes should be investigated through future studies.

But Padmanabhan believes his team is building evidence of the importance of mental health considerations for doctors treating heart patients, by exposing these areas in need of insight.

"There could be some people who are predisposed to depression who we should not be giving these drugs," he said. "We need to now validate this by showing the mechanisms by which this has an effect."

In contrast, some people with depression could benefit from these drugs. "If angiotensin blockers are protective, then there is a role to repurpose them," Padmanabhan said. Repurposing would be significantly faster than developing new antidepressants, as the drugs will have already passed safety tests for use in humans.

"This will not change clinical practice immediately," Padmanabhan said.

But he highlighted that as blood pressure medication is prescribed as a preventive measure against stroke and heart disease, most people take them for prolonged periods of time, during which they could be increasing -- or decreasing -- their risk of depression, depending on which drug they are prescribed.

Thinking twice about the best drug to take

"It's an interesting study that makes us think about the choices we make when treating hypertension in patients," said cardiologist Dr. Nieca Goldberg, medical director of the Joan Tisch Center for Women's Health at NYU Langone Medical Center. ACE inhibitors "are very good at lowering blood pressure ... and this confirms they are good choice for people with mood disorders."

Goldberg, who was not involved with the study, said she already monitors for mood disorders in her patients and highlights that, although beta blockers were shown to increase risk of depression, they are a very effective drug for other heart conditions such as angina or certain arrhythmias. "It means you just have to monitor patients a little more closely for symptoms of mood disorder," she said.

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Goldberg also pointed out that the study observed patients older than age 40, which means it is unlikely that many, or any, were pregnant. "You can't extrapolate this information to pregnant women, who would be younger."

The researchers at the University of Glasgow now plan to investigate this further, to help understand the links in all subsets of the population.

"If there is an effect, we should do something about it," Padmanabhan said. "Understanding more about drugs and mental health will help us tailor the right medicines to the right patients."

The British Heart Foundation (BHF), who helped fund the research, agreed.

"There may have been other factors influencing the outcomes therefore further study in this area is warranted," said June Davison, a senior cardiac nurse at BHF. "It's important that health professionals are aware of the possible impact that high blood pressure medication can have on someone's mental health. If you are on blood pressure medication and concerned about your mood, have a chat with your GP."