UPDATE 2-US FDA approves Lilly, Daiichi's blood thinner

* Warning about bleeding risk may slow use -analysts

* FDA: companies must study cancer cases

* Lilly says will be "competitive" with Plavix

* Lilly shares close up 1 pct (Adds FDA and company comment, analyst reaction, background)

By Susan Heavey and Lisa Richwine

WASHINGTON, July 10 (Reuters) - U.S. health officials approved Eli Lilly and Co (LLY.N) and Daiichi Sankyo Co Ltd's (4568.T) blood thinner Effient after several delays, and called for the pill to carry a strong warning about bleeding risks.

The drug, a rival to Sanofi-Aventis SA (SASY.PA) and Bristol-Myers Squibb Co's (BMY.N) blockbuster drug Plavix, will include a "black box" warning about its potential for "significant, sometimes fatal, bleeding," the Food and Drug Administration said on Friday.

Both drugs aim to prevent dangerous blood clots that can cause heart attacks or strokes. Studies showed Effient was more effective at preventing heart problems than Plavix, but had a higher risk of bleeding.

"This is a drug that does some very good things - it's a very important part of the therapeutic landscape - and it has some things you've got to watch for," Dr. Robert Temple, head of the FDA office that reviews heart drugs, told reporters.