When transitioning from citalopram 15mg to sertraline 25mg, it is important to do so gradually and under the guidance of a healthcare professional. You can either switch directly or gradually taper off one medication while starting the other simultaneously. Consult with your pharmacist to create a personalized tapering schedule and ensure a smooth transition to the new medication. It is not recommended to abruptly switch medications in most cases. Listen to your healthcare provider's advice and follow their guidance closely during the transition process.