Depending on your annual income, you have a monthly limit of €8, €18 or €60 (these last limit applies only if your pension exceeds 100,000 euros per year.
It seems that every regional government will use different ways to apply the limit. In Andalusia, they say the system is prepared to stop charging you when your limit is reached. Others say they will refund the excess over the limit in a few months. If you live in Santiago, you should check what the Murcia government is doing.
If you are entitled to the €8 limit, make sure you follow whatever procedure has been set up by your regional government to get the excess back.
Most pensioners are expected to be withitn the €18 monthly limit. If this is your case, then you will have to pay €15 per month up to €18 if you are prescribed further meds in the same month.