Many lenders in the US  do permit additional payments to be made against the principal. Those will reduce how much you actually owe, which reduces the total interest you will owe over the duration of the loan as well as shortening its duration. It is not uncommon for people to take a 30 year mortgage so they have the option of paying slowly, but to make the additional payments to shorten it to 20 or 15 years... at a total cost not too much more than the shorter loan would have cost.


Not all lenders/loans will let you do this. If yours doesn't, your only option to reduce total cost of the loan is to refinance.

Note that you may need to explicitly say you want the extra applied to principal, or they may count it as just prepaying future installment payments the loan.