The FDA doesn't regulate off-label use per se, but does [well, try to] regulate the advertisements for off-label use. Basically manufacturers are not "allowed" to do the latter, even though clinicians can still prescribe off-label. As an FDA document explains:
the modern Federal Food, Drug, and Cosmetic Act (FD&C Act) and FDA
regulations prohibit manufacturers from introducing new drugs and most class III medical
devices into interstate commerce for any intended use that FDA has not determined to be safe
and effective. These authorities also prohibit manufacturers from introducing into interstate
commerce devices subject to premarket notification requirements under section 510(k), which
includes most class II and some class I devices, for any intended use that is outside FDA’s
substantial equivalence determination (clearance) for such devices. Devices that are exempt
from premarket notification requirements, generally because they are low risk, may be
introduced into interstate commerce for the specifically exempt intended use(s) without
obtaining FDA clearance. To establish a manufacturer’s or distributor’s intended use for the
product, FDA is not bound by the manufacturer’s or distributor's subjective claims of intent, but rather can present objective evidence, which may include a variety of direct and circumstantial
Under the FD&C Act, an approved new drug that is accompanied by written, printed, or graphic
matter that suggests an unapproved use may be an unapproved new drug with respect to that
use. Furthermore, an approved prescription drug that is intended for an unapproved use
(whether referenced in labeling or not) would be considered misbranded, because the drug does not meet the regulatory exemptions from the requirement that its labeling bear “adequate directions for use.” Similarly, a medical device that is intended for an unapproved use is
considered adulterated and misbranded.
It then goes on into a pretty long discussion of Section 401 of FDAMA and how it was challenged in courts on First Amendment basis. However that provision sunset in 2009. So the FDA now has pages and pages of guidance (like the document I linked to) what the "best practice" for the manufacturers is with regards to off-label information, e.g. what off-label claims they can make in scientific journals, textbooks etc. For example, if you are a manufacturer and pretend to put out a "textbook" that is mostly advocating some off-label use for your product, you fall foul of FDA's guidance.
It's not clear to me what the implications are for falling foul of these, other than something like "the FDA is really pissed off with you, [so] your next new drug application might take 250 years to complete".