When you apply for health with health costs through the LIS, your entitlement starts from the date your HC1 application is received.
Any help you're entitled to cannot be backdated to cover health costs you've had in the past.
If you've received an enquiry letter or Penalty Charge Notice (PCN) and did not have a qualifying benefit or exemption at the time, you must pay the charges.
Check what help you could get to pay for NHS costs and apply online.