Hi, my boyfriend has a fatty liver and his doctor has prescribed him sertraline (zoloft) for anxiety how big of a risk will this affect his liver?

Minimal probable: Depending on the severity of your BF's liver disease, the medication Sertraline should be ok, but his Liver function tests should be monitored periodically as a precaution. Best wishes.

Monitor LFT's: If his LFT's have been stable, your boyfriend should get periodic bloodwork to monitor for changes in his LFT's. If he shows any signs of liver damage, zoloft (sertraline) should be discontinued. If they remain stable, he can continue the zoloft (sertraline) for the anxiety.