This really depends on the illness, the medicine, and the patient.
Antibiotics are definitely overprescribed in some societies.
In many cases of a “simple cold”, it is better to take nothing (antibiotics will not act against the cold virus) and rely on your own immune system.
So why are antibiotics prescribed against some of these diseases? Because, in many cases, the viral infection (for which the antibiotic has no effect) can be accompanied by a bacterial infection (against which the antibiotic will be effective). So the antibiotic is given to reduce the probability of additional infection. This is especially important if the patient has a compromised immune system, or other health problems, such as asthma or other lung disease.
In all cases involving illness, it is best to have a doctor who is familiar with your specific situation handling the case. Make especially sure that you have with you a detailed list of all medications that you take regularly, along with their dosages. The help that the doctor can give you is directly related to the amount of information that he/she has about you.
I hope that this has been of some help.