If alcohol and drugs seeps into breastmilk how come painkillers don’t?

I’ve avoided taking anything that isn’t food into my system, to ensure my baby doesn’t get anything that isn’t nutritious for her, via my milk. However, I suffer from rheumatoid arthritis and fibromyalgia and I am and have been in so much pain physically. I’ve done yoga/Pilates and physio, but it hasn’t relieved or eliminated the pain. I struggle to sleep or sleep for longer than 20mins because of the pains. But I just persevere through.

Today doctor said I could still take pain killers and co codamol / dihyrocodein. How is this ok to take when surely what we eat goes into the milk production process?