If you you have regular medication to take; ensure that you have enough for at least twice the length of your stay. Take some on carry on and some in the hold of the aircraft. If your medication requires to be kept cool let the hotel know that you will require a fridge in the room. If staying in a dorm this will be more of a challenge. Keeping medication in not secure areas is not recommended.
Ensure that the medication has YOUR name on it and a letter from your doctor will be useful.
Phnom Penh and Siem Reap have major international hospitals and can deal with most medication requirements.