There's a new choice for treating erectile dysfunction, but the question investors should be asking is whether it'll increase sales of Eli Lilly's (NYSE: LLY) Cialis. The current dosage of Cialis has a 36-hour shot clock in which it works, but now the FDA has approved a low-dose version of the drug that can be taken once daily.

The ironic thing is that Eli Lilly only needed to get the approval because its drug lasts longer than its competitors'. Pfizer's (NYSE: PFE) Viagra and Levitra, which is marketed by GlaxoSmithKline (NYSE: GSK), Schering-Plough (NYSE: SGP), and Bayer, can be taken daily because they don't last as long, although there's no Food and Drug Administration approval for it.

The big question is whether the new dosage will increase sales of Cialis. Sales of the drug will likely go over $1 billion for the first time in 2007, but it still trails Viagra in total sales. The drugs may make patients blind or deaf, but men are still going to take them.

Eli Lilly is going to price a one-month supply of the once-a-day dose at about the same price as eight of the tablets that last 36 hours, so it may be that patients who are having sex more than twice a week will switch to the low-dose formula without any increase in revenue.

Whether Eli Lilly can convince men and their partners who like the idea of spontaneity to switch from rival products remains to be seen. But I can't wait to see the new advertisements.

Eli Lilly and Glaxo are picks of the Income Investor newsletter. If you'd like to see more ideas for stocks that pay dividends, grab a 30-day free trial and check out all of our past recommendations.

Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Pfizer is a pick of the Inside Value newsletter. The Fool has a disclosure policy.