Topical minoxidil is an FDA-approved hair loss medication that does not require a prescription whereas oral minoxidil is considered an off-label prescription for hair loss and does require a prescription. "Off-label" means it's not FDA-approved for that particular use, but can still be prescribed at the provider's discretion.
Topical and oral minoxidil contain the same active ingredient. While there have not been clinical studies comparing their efficacy, there appear to be some benefits of using an oral treatment method vs. a topical medication method, such as more precise dosing and better medication adherence & consistency.
Using both topical minoxidil and oral minoxidil at the same time will not produce better results and therefore, is not recommended.