Hello from JustAnswer.
There are several types of local reactions that can occur at injection sites. The most common are due to local irritation from the drug or an allergic reaction. These reactions happen relatively quickly and are usually best treated with the use of oral antihistamines
and the application of moist heat to the area, such as with moist heating pads or hot compresses.
It is also possible to develop a local infection, particularly if needles or the injection fluid were not sterile or shared between individuals. These reactions tend to occur later, as it takes some time for germs to grow and cause the infection. These types of reactions are also harder to treat at home, since there are no good antibiotics available over the counter. It is better to use systemic
antibiotics, so it would be better to be seen by a doctor. You can try applying an over the counter antibiotic ointment until you can get to a doctor, but this is usually not effective.
So, a quicker reaction can be treated with antihistamines and moist heat, but a delayed reaction would be better treated with antibiotics by your doctor.
If I can provide any further information, please let me know.