The Food and Drug Administration on Friday approved Weltruza, a once-a-month shot for adults with schizophrenia. The drug inside is olanzapine, a pill that has been used since 1996 and is one of the most prescribed medicines for the illness. Drugmaker Teva said the new form is given under the skin and slowly releases the drug over a month.

Why does that matter? Many people with schizophrenia have trouble taking a pill every day. Missed doses can lead to relapse and hospital stays. A monthly shot is meant to make treatment easier to keep up. Teva estimates about 2.1 million people in the U.S. have been diagnosed with schizophrenia.

Teva said patients will not need to be watched by staff after each shot. They also will not need a starting "loading" dose or extra pills. The company plans three strengths, 318, 425 and 531 milligrams, matching daily pills of 10, 15 and 20 milligrams. It said they will be available in the coming weeks.

The watch period is the key point. An older monthly olanzapine shot, Zyprexa Relprevv, has been sold since 2009. It carries a risk of a rare reaction that can cause heavy sleepiness or confusion within hours, so patients must stay at the clinic afterward. Fierce Pharma reported that this rule has held back that drug's use.

In Teva's main study, called Solaris, patients on all three doses improved on a standard symptom scale after eight weeks. No cases of that rare reaction were seen. Still, the label lists it as a possible risk, and Teva said it is not known whether it can happen with Weltruza. Common side effects included weight gain, sleepiness, headache, constipation and soreness where the shot was given.

The drug uses delivery technology from French partner Medincell, which also helped make Teva's monthly risperidone shot, Uzedy. Price has not been announced.

What we know: The FDA approved Weltruza on Oct. 9, and Teva plans to launch three doses within weeks.
What we don't: The price, and whether the rare post-shot reaction can occur with the new form.