A coxsackievirus virus infection can last for a variable period of time. Most cases will resolve within 3-5 days, but in some people it can last up to 2-3 weeks. It is more likely to last longer or be more severe in adults.
One of the possible manifestations of a coxsackievirus infection is as a bronchitis. It is also possible for someone with a viral bronchitis to develop a bacterial secondary infection, and that is why your doctor prescribed an antibiotic. Since the antibiotic did not help, it is most likely that the cough and sputum production is a result of the coxsackievirus causing a viral bronchitis. Chest pain and fatigue are also common symptoms in someone with a viral bronchitis.
With a viral bronchitis, there is no direct treatment, so the usual approach is to use medicines to ease symptoms. If you have been taking plain Robitussin and it has not been effective, then it would be appropriate to try a cough suppressant, such as dextromethorphan, or a decongestant, such as phenylephrine. Some of the formulations of Robitussin will include these medicines, or they can be taken separately.
If you develop shortness of breath, then that would be more of a concern, and may indicate some spasm of the airways. If so, it also may help to use an inhaler to relieve the spasm, such as albuterol, but this is only available by prescription, so it would be necessary to be seen by your doctor.
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