Definition
The 24-hour window (customer service window) in the WhatsApp Business API starts with the last message the patient sent you. During that time you can send any free-form text, photo or document.
Once the window closes, you can only reach the patient with a template approved by Meta. When the patient replies to the template, the window opens again.
Why it matters for clinics
Teams that don't know this rule lose time trying to message closed conversations, or put off follow-ups. Knowing when the window closes means important information (price, treatment plan, next step) gets sent while it's still open. It's another reason a short first-response time matters.
Example
A patient sends photos at 10pm. The agent sends the assessment and treatment plan at 10am the next day, while the window is still open. When the patient hasn't replied after three days, the agent uses an approved follow-up template.