Now that the weather has warmed up, Phrosties — a boozy slushie sold and delivered over social media — have become an under-the-radar hit in the city.

But the frozen, sweet, technicolor hooch — which costs $10 a bottle — is “unregulated,” according to the State Liquor Authority, and the agency has launched an investigation, The Post has learned.

“The SLA is looking into the illegal sale of this unregistered and potentially dangerous alcoholic drink‎ by an unlicensed business,” Bill Crowley, the authority’s director of public affairs, told The Post.

To order one, customers send a text to a phone number on the company’s Instagram page, give their own Instagram handle and their address, then wait for delivery.

Workers with coolers full of Phrosties, which come in plastic bottles, arrive in a couple of hours.

Not even customers know the drink’s ingredients — but it’s knock-you-out strong, one food reviewer said.

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“After the first sip, you’re loving the sugar. After the third sip, you’re thinking how pretty the colors look and why can’t you feel your lips? After chugging the whole thing, who cares? You’re lying blotto on the sidewalk and all is bliss,” New York magazine food critic Adam Platt wrote after guzzling one.

“This is just pure sugar and grain alcohol. It tastes like Kool-Aid-meets-Red Bull, mixed with 150-proof Everclear.”

On its Instagram page, the company touts the drink as, the “BEST TASTING FRUIT SLUSHIES IN NYC.” The company has roughly 16,000 Instagram followers.

Flavors include Irish Bomb, Volcanic Paradise and Tsunami Sunrise.

The service has been around for roughly a year but has became more popular in recent months as word spread and the weather warmed.

It’s hard for the SLA to regulate the shady business, because it’s unregistered, Crowley explained.

“We are worried about this, but they’re dealing in illegal trafficking … It’s in the hands of local authority, the NYPD,” he told the New York Observer.

A worker for Phrosties refused to deliver to a Post reporter on Sunday, saying, “We don’t have anyone in your area. Sorry.”

Additional reporting by Frank Rosario.