Not all medical issues can wait. In fact, a lot of them happen at the most inconvenient times. Luckily, there are a number of medical centres (‘centros de salud’) which are open in the evenings and at weekends.
Of course, arriving without an appointment, you are likely to have to wait to be seen so arriving as early in the day as possible is advised.
Unfortunately, none of these centres are located in the centre but they are on bus routes or within a 20-minute taxi ride: