Medical bills that have been sent to collections can impact your credit score a lot.
When I had a medical bill sent to collections because the health insurance wouldn't take care of it the medical bill that was sent to collections dropped my credit score by 15 points in the beginning and now it's dropped another 15 points.
I would pay the medical bill if I could but I just can't afford it so it's just gonna have to impact my credit score but overtime it should go away anyhow within 7 to 8 years.
I don't use credit much anyway so it doesn't hurt me as much as it would others who depend on credit to get loans, mortgages, car loans etc.
If possible try to work out payment arrangements with the hospital or clinic before the medical bill gets sent to collections but if it gets sent to collections then paying it off won't improve your credit score anyway.