Eli Lilly (NYSE: LLY) received an especially harsh rebuke from the FDA on Thursday about one of its top candidates. The agency refused to approve a long-lasting injection (LAI) version of the company's multibillion-dollar Zyprexa for schizophrenia and bipolar disorder.

In a surprise move, the Food and Drug Administration issued a non-approvable letter for the injection because of questions about the "excessive sedation events" seen in patients taking the drug. Rather than issue an approvable letter or a narrow label for the drug, the FDA ignored the advice of a somewhat positive advisory panel that had voted in favor of approval.  

The rejection of Zyprexa LAI is unfortunate for Lilly. It would have had a very strong competitive advantage versus other atypical antipsychotic drugs because of its superior every-four-week dosing. This long-acting treatment matters because schizophrenics often don't follow recommended dosing, and with it, it's relatively easy to see if a patient is sticking with his or her therapy.

There are already several long-lasting versions of the less desirable typical antipsychotics on the market. Johnson & Johnson's (NYSE: JNJ) and Alkermes' (Nasdaq: ALKS) Risperdal Consta, which had sales of $1.1 billion last year, is the only long-acting atypical antipsychotic drug on the market now, and its injections are effective for two weeks.

Other drugmakers, like Vanda Pharmaceuticals (Nasdaq: VNDA), are also developing four-week-long injectable versions of atypical antipsychotic drugs, but Lilly's Zyprexa LAI was the only other long-acting atypical one with the opportunity to be approved soon.

Zyprexa LAI could have easily become a blockbuster therapy for Eli Lilly because of the huge need to improve many schizophrenics' compliance with therapy. The consolation prize for Lilly investors is that the FDA non-approvable letter won't affect sales of its Zyprexa tablets that are taken daily. But Zyprexa sales may come under pressure in the second half of the year as Johnson & Johnson's Risperdal starts to face generic competition.

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article. The Fool has an A+ disclosure policy.