You are correct that Avastin is an FDA approved chemotherapy drug used for colon cancer. It works by blocking the proliferation of tumor blood vessels. Its use in macular degeneration is considered "off-label" meaning that it has never been approved by the FDA for macular degeneration treatment.
Wet macular degeneration is caused by abnormal blood vessels growing underneath the retina. Since Avastin blocks blood vessel growth in colon cancer, the thought is that it works to block the abnormal blood vessel growth in wet macular degeneration.
Though it is not FDA approved for macular degeneration, it is widely used for wet macular degeneration and is considered standard treatment. The same company that makes Avastin reformulated the molecule to a smaller fragment and did receive FDA approval for macular degeneration. That medication is called Lucentis
The dilemma is that both Avastin and Lucentis works in a similar way to block abnormal blood vessels and have been safely used for many patients. Avastin is not FDA approved for the eyes while Lucentis is FDA approved. The cost difference is extreme however, with one dose of Avastin being a few hundred dollars while Lucentis can be $1500-$2000 per dose.
From experience, Avastin and Lucentis has shown equal efficacy. We will not know which is better however until data from a large nationally funded head to head trial is published. The study is currently underway.
Rest assured that Avastin is widely used and is standard eye treatment at this time.Please remember to press the accept button if my answer so I can receive credit for assisting you.