Only certain conditions on an official list are paid for 100%. They include treatment for cancer, heart conditions etc.. Otherwise drugs are covered solely on the basis of 'La Basse' which means that you pay a percentage over what the government does.
Once you have retired 'La Basse' is reimbursed by the UK Government. The rest is up to you. The increasing cost of health-care was a consideration in our decision to move back to the UK this year (after 15 & 20 years).
If you don't take out a 'complementaire' insurance you have to pay all the excess over La Basse - and the rules are getting tighter all the time.
Example - I was 65 and my 'complementaire' had gone up to 76 euros a month. Even so things like spectacles (horrendously expensive, cheaper to come to UK and go to Specsavers) will only cover up to about 300 euros. Dental treatment also leaves a big gap which you have to fund. Mr. J2 could no longer afford to pay. His treatment for cancer was covered once it was diagnosed, but until then we accrued bills of about 2,000 euros.
It all depends on what your prescription is for. Please check it out.