is there a minimum interest rate

No, but there is a maximum under federal law of 45%. You don't want to be charged with loan sharking.

There are usury laws in Massachusetts can be complicated and are triggered at a rate far less than 45%. If you wish to charge in excess of a certain rate in Mass. then you need to notify the Attorney General. If the loan did not include an interest rate, you cannot go back and change the agreement. If you are planning to give a loan, then this should be included in any documentation.

As a guideline, the legislature set the interest rate at 12% for judgments. If you were to sue the person to pay you, this is the rate collected until paid, generally.

DISCLAIMER: This answer is provided in response to a "hypothetical" question and provided for general, informational purposes and does not create an attorney/client relationship. The information presented is not legal advice and may change based additional information and research. It is recommended that you speak to an attorney to discuss your specific legal issues.