You can deduct the interest you pay on a loan when you borrowed money to invest. If you put a mortgage on your home and put that money into the stock market, then the interest on that mortgage is deductible as long as you have kept the money in the market. If you have a rental property the interest on the mortgage is a deduction, if you got the mortgage to buy the property. If you have a rental property and you put a mortgage on that property to pay off your personal credit card balances, then the interest is not deductible. What you used the borrowed money for is the important part about the borrowed money.
If you are using a credit card to buy items for your business, then the interest on the credit card will be deductible. This will only work if all of the charges on the card were for business, or if you prorate the interest based on the portion of the charges on the card that were for business compared to the portion that were personal. This process can be annoying, so if you can, use one card for business and one for personal.
You will only get a deduction for interest expense from CRA if you can prove that the money was borrowed for business purposes.