Medicines with prescription as first time arrival in KSA

As the other poster said, check the medicine list and if you are carrying any medicine on that list, you should have a prescription.  Plus what most people don't know that "theoretically" this prescription needs to be attested by the Saudi embassy in your home country for it to be valid and considered.  Valid only for customs not for getting more medicine from the pharmacies here - for that, you need to consult and get a prescription from a doctor here.

Practically, if it is a small quantity of medicine, no one bothers to check.  It also depends on WHAT that medicine is.  Medicines with potential of misuse like Tramadol and / or slightly more famous ones like codeine are more likely to be frowned upon IF you are stopped at customs.