A long-delayed announcement from the FDA was even more positive than expected.Axsome Therapeutics (AXSM) stock recently shot higher on a generally lousy day for the overall stock market. The gains came in response to the FDA's approval of Auvelity, the first new line of treatment in years for millions of Americans with major depressive disorder.Does the FDA's approval of Axsome Therapeutics' first drug make this a smart stock to buy now? Let's weigh the reasons to buy against the reasons to remain cautious.Reasons to be cautiousIt's been 15 months since the FDA began an expedited review of the drug formerly known as AXS-05 that was supposed to be much shorter. First, there was a common manufacturing issue the company needed to overcome.Auvelity's approval was also held up longer