Health messaging through the media can be both positive and negative.

Yes, this is true that health messaging thought the media can be both positive and negative.
Let's take a positive example: the message that we should do sports, which can be found for example in gym commercials is a positive message.
On the other hand some messages might be biased, especially if they are for example sponsored by a company that wants to sell a product - this would be an example of a negative effect of such messages