insertAdjacentHTML() is more efficient than innerHTML when adding content because it only parses and updates the specified position without rewriting all the element's contents. innerHTML completely replaces the content, forcing the browser to reparse everything. insertAdjacentHTML() accepts two parameters: the position ('beforebegin', 'afterbegin', 'beforeend', 'afterend') and the HTML string to insert. However, like innerHTML, it still needs to be used carefully with sanitized input to prevent XSS attacks.