TORONTO - Health Canada says it is reviewing the antidepressant citalopram after a new study suggested high doses -- 60 milligrams a day -- can affect the electrical activity of the heart.

Health Canada says the changes in electrical activity could potentially lead to serious and possibly fatal abnormal heart rhythms.

The drug regulator says patients using the drug should not stop taking it and should not alter their dose on their own.

It suggests patients can talk with their health-care provider if they are concerned and should notify their doctors if they feel any side-effects such as dizziness, difficulty concentrating, feelings of anxiety, nausea or vomiting.

Health Canada says people taking the drug should seek immediate care if they experience an abnormal heartbeat, shortness of breath, dizziness, or fainting.

The drug is sold by a number of brand names, including Celexa; it is in the family of drugs called selective seratonin reuptake inhibitors or SSRIs.

The current recommended dose in Canada is 20 milligrams a day for adults, though people who don't respond to that dose are sometimes prescribed doses of 40 and even 60 mgs a day.

People with heart conditions or a history of low levels of magnesium or potassium in the blood may be at higher risk of developing an abnormal heart rhythm while taking citalopram.