P O is by mouth
I V is intravenous
I M is intramuscular by needle
SubQ is just under the skin by needle
topical is absorbed through skin (by cream/ointment/patches)
some meds can also be inhaled
and yes, meds can even be administered through the rectum
It all depends on what type of medication you are taking or need to administer. Many medications are in pill form and could take it by mouth, as well as liquid forms of medication. Some medication requires administration via needle or in some cases, there are medications that are administered in the anus (although these are rare).