When I was on lithium it was an extended release. I liked it until I realized I'd gained almost 50 lbs. weight gain is a deal breaker for me too MM, as you know.
But I guess not everyone gains weight on it. Oh OP asked about side effects.. Initially I had a salty taste in my mouth and wanted to drink water non stop. Pdoc had to tell me to slow down on the water - i guess i was drinking too much water - who knew right lol? But those side effects went away fairly fast.