Private lending in Canada sits on top of three federal statutes, and a document that ignores any of them can be cut down or struck out. The first is the Bills of Exchange Act (RSC 1985, c. B-4), whose Part IV defines the promissory note and fixes its essential elements: the promise must be unconditional, in writing, signed by the maker, and for a sum certain payable to a specified person or to bearer. A note that makes payment conditional on some uncertain event, or that fails to state a definite amount, is simply not a valid note under the Act, and the lender loses the procedural advantages a clean instrument carries. You can read the statutory definition in full through the federal government's authoritative consolidation at the Justice Canada text of the Bills of Exchange Act on promissory notes.
The second is the Interest Act (RSC 1985, c. I-15), and section 4 is the trap that catches careless lenders most often. Where interest is expressed on any basis shorter than a year, a monthly or weekly rate, and the contract does not also state the equivalent yearly rate, the lender can recover no more than five percent per year, no matter what number the parties actually agreed. A note promising "1.5 percent per month" with no annual equivalent is a note effectively charging five percent. The fix is simple but non-negotiable: always express the rate as an annual percentage.
The third is section 347 of the Criminal Code. Charging interest above the criminal rate is an offence, and that ceiling was lowered as of 1 January 2025 from an effective annual rate of sixty percent to an annual percentage rate of thirty-five percent, with the calculation method changed from EAR to APR and the definition of interest widened to capture most fees and charges. For an ordinary loan between individuals the practical lesson is that everything you load onto the borrower, the headline rate plus penalties and administration charges, has to stay under that APR cap when totalled. Loans to corporations and very large advances sit under different exemptions, but a personal loan does not.