Officers who carry pain-relief medication can use the same Medication Register as patrols, straight from the Field view.
Open it
Tap the menu (☰) and choose Medication Register. It only appears when all three of these are true: your organisation uses the medication register, your role includes pain-medication access, and the unit you're signed on as has the register turned on. If you need it and can't see it, ask your administrator.
Use it
Everything works exactly as it does in the Patrols interface: pick your vault, check current stock, and use Add Record for a stock check or a movement. See:
Using the Medication Register
Recording a Medication Stock Check
Recording a Medication Movement
When you administer a vial in the field, record it on the patient's first aid log (methoxyflurane section, Vial Number) so it's marked as used. See Recording a First Aid in the Field.