Hello Kasia79, I have been getting Botox in my forehead and crows feet for 10 years (every 4-6 months), sometimes I get the flu like feeling that last 2-5 days after, and sometimes I don't. If I do get it, it does go away. Because I it's inconsistent, this leads me to believe it may be watered down at times (when I don't get sick). Or when I do get sick it's "old". Botox does have a short shelf life.