U.S. regulators Thursday approved the first drug designed to prevent chronic migraines.

The Food and Drug Administration's action clears the monthly shot Aimovig for sale. It's the first in a new class of long-acting drugs for preventing migraines. Three other shots are expected to win approval by next year, and several pills for preventing migraines are being tested.

Current prevention treatments include pills originally developed for epilepsy and other conditions and the wrinkle reducer Botox, but many patients abandon them because they don't help much or cause serious side effects.

Amgen Inc. of Thousand Oaks, California, and Swiss drug giant Novartis AG developed Aimovig. Injected monthly just under the skin using a penlike device, the drug will cost $6,900 per year without insurance.

Migraines can cause disabling symptoms: throbbing headaches, nausea and vomiting, and sensitivity to light and sound. About 10 million Americans get them frequently. They're most common in people in their 30s, mostly women, and can last for several hours or even days.

In one study, patients given Aimovig saw their migraine days cut from eight to four a month, on average. Those who got dummy shots had a reduction of two. Each patient group had similar minor side effects, mostly colds and respiratory infections.

Some patients saw their migraines completely eliminated, said Sean Harper, Amgen's research director.

Aimovig and the migraine drugs in development target a substance called CGRP whose levels spike in the blood during a migraine, triggering symptoms.

The long-term safety of Aimovig, also known as erenumab, hasn't been tested, and Amgen plans to track outcomes in women who become pregnant while taking it.