Eli Lilly and Co. won approval from U.S. regulators for a long-acting, injectable form of its bestselling antipsychotic Zyprexa, the company announced Monday morning.

Zyprexa Relprevv, when injected into a patient’s muscles, lasts up to four weeks. The U.S. Food and Drug Adminstration OK’d the long-acting version to treat schizophrenia.

The drug could bring some much-needed profit to Lilly. The Indianapolis-based drugmaker will lose most of the $4.7 billion a year Zyprexa now generates in revenue when cheaper generic versions of the drug hit the market two years from now.

But the long-acting version of Zyprexa has patents that could run until 2018. Analysts have been predicting no more than modest revenue from the new version, since the FDA had previously declined to approve the drug over concerns about its side effects.

But it could be significant if Lilly can duplicate the success competitor New Jersey-based Johnson & Johnson has had with its injectable schizophrenia drug Risperdal Consta. The drug has achieved sales of $1.3 billion a year after many patients who were taking Risperdal pills switched over to the longer-acting version.

Long-acting medicines for schizophrenics are beneficial because many patients don’t remember to take a pill each day and caregivers can struggle to administer the medicine on a daily basis.

"There is a growing recognition among psychiatrists in the United States that non-adherence to medication is an even greater barrier to care for patients with schizophrenia than was previously understood, and that long-acting treatments can play a beneficial  role in helping patients maintain a stable treatment regimen," said Dr. John Hayes, vice president of Lilly Research Laboratories, in a prepared statement.

The FDA initially said it could not approve long-acting Zyprexa because of concerns about it causing excessive sleepiness in some patients. But Lilly worked with the FDA to develop a mandatory patient care program, which restricts distribution of Zyprexa Realprevv to medical professionals or patients enrolled in the program.