Is there any free to use XSD (DTD) for Rx documents (drug prescription)

I'd like to explore and possibly use (reuse) open XML formats/schemes (DTD/XSD) for drug (medication) prescription documents (Rx) issued by medical doctors and later processed by licenced pharmacists and health insurance companies.

Is there any such freely XSD available and what I have to take care when considering and incorporating into health-care information system?

What international standards (ISO, ISO/IEC, EN) should be used to acheeve maximum level of interoperability?

From stackoverflow

• please provide more information about what you want to achieve