The medicine is not for internal use.

involving or affecting only the people who are part of a particular organization rather than people from outside it

The company has launched an internal investigation into the claims.

an internal memo/document

Does the bank to have adequate records, systems and internal controls?

the internal workings of government

internal divisions within the company

connected with a country’s own affairs rather than those that involve other countries 

He accused the US of trying to interfere in his country's internal affairs.

an alleged threat to the country's internal security

coming from within a thing itself rather than from outside it

a theory that lacks internal consistency

Some photos contain internal evidence that may help to date them.

She struggled with her own internal conflicts.

LEARNit Dictionary