http://www.nhs.uk/Conditions/Sore-throat/Pages/Treatment.aspx
The use of antibiotics (medication to treat bacterial infection) is not usually recommended for the treatment of sore throats. This is because:
Most sore throats are not caused by bacteria.
Even if your sore throat is caused by bacteria, antibiotics have very little effect on the severity of the symptoms and how long they last, and may cause unpleasant side effects.
Overusing antibiotics to treat minor ailments can make them less effective in the treatment of life-threatening conditions.
Antibiotics are usually only prescribed if:
your sore throat is particularly severe
you are at increased risk of a severe infection, for example because you have a weakened immune system due to HIV or diabetes (a long-term condition caused by too much glucose in the blood)
you are at risk of having a weakened immune system, for example because you are taking a medication that can cause this, such as carbimazole (to treat an overactive thyroid gland)
you have a history of rheumatic fever (a condition that can cause widespread inflammation throughout the body)
you have valvular heart disease (a disease affecting the valves in your heart, which control blood flow)
you experience repeated infections caused by the group A streptococcus bacteria