The previous potions that Chloe/Claude has been taking stunted his growth and made it so that his physique could come off as a girl I think. Now he asked for the opposite - so Im guessing it will let him turn into what his body would have normally been if he didn't take the previous elixir at all
The side effects Blair was talking about was because the potion is literally a poison. Blair thinks that Chloe is just using it to seem sickly and stunt her growth. She's is worried that if Chloe keeps taking it then her health would suffer as a result. She doesn't know that Chloe is a boy yet.
Wait wait whats the effect of the elixir again ( ̄∇ ̄")