Yes. Phase one human trials have been completed and confirmed to be safe for humans. Many human studies since then have been conducted and are ongoing.
No one can give a guarantee that increased NAD would have no effect on tumour growth. But, in all studies to date, there has been no increase in frequency or severity of cancer. Humans have “cancer” cells in our bodies constantly, but usually our immune system is able to stop them until we get older. Diminished NAD as we age is a big hinderance to fighting infections and cancer, so there is reason to believe increased NAD will help fight cancer more than it will help cancer cells grow.