The chemist, known as a farmacia is the first port of call for most Spanish people who feel unwell. The pharmacist are usually very helpful and will often be able to diagnose problems and treat them.
Chemists are denoted by a flashing green cross hanging out over the street, therefore not difficult to see. If the green cross is not flashing then the Chemist is shut.
Chemists generally open during normal shop hours but, as stated above, there is usually one or more, chemists in each town which are open out of hours, including Sunday mornings.
A notice, hanging on the door or window of every chemist, will give you the location and times of the duty chemists in the area.