What's the worst side effect you've has after using a skincare product?

The worst is Jiore product. Exactly opposite reaction of what each product claims.

I had a Staph Infection when I was using Nivea moisturiser! It makes your skin oily. For somebody who already has oily skin, has a beard, and used to ride a bike on ORR in Bangalore with a helmet, this combination led to boils on my face. While I realised this and stopped using it, staph infections don't go that fast and recur frequently. Finally, WFH was the solution. So, it was more of a second order effect of using Nivea moisturiser and then realised not all products suit everyone!