Many small businesses and entrepreneurs are using tweets to market their products.
But the new ruling from the Federal Trade Commission makes it a crime if claims are not backed by specific facts.
This ruling will make the marketing of products, services, or offerings harder.
For example, whether it is including the average effectiveness of a weight-loss shake or noting that a celebrity was paid to push a product in a Twitter post, marketing companies need to apply the same standards to online ads as they long have to older media.
That means making room for full disclosure even in a 140-character tweet on Twitter.
The agency suggested that marketers could flag Twitter ads by including "Ad:" (three characters) at the beginning of the post or the word "sponsored" (nine characters).
The new guidelines are important because they suggest the grounds on which the agency might open an investigation.
The report expands the FTC's more than decade-old rules on Web ads to the world of social media and smartphones.
The FTC said marketers need to be conscious of the location of disclosures and ensure that users can still see them easily on a smartphone.
If a company can't find a way to make its disclosure fit the constraints of social or mobile ad, it needs to change the ad copy so that it doesn't require a disclosure, the agency said, making that point explicit for the first time.
Disclosures must be clear enough that they aren't "misleading a significant minority of reasonable consumers," the FTC said.
The FTC provided some hypothetical promotions that could cross the line. One would-be Twitter post from a fictitious celebrity account said: "Shooting movie beach scene. Had to lose 30lbs in 6 wks. Thanks Fat-away Pills for making it easy. Typical loss: 1lb/wk. #Spon." The FTC said the used abbreviation for sponsor, "#Spon" could be unclear to consumers.