Well heres the answer.
from
http://www.medscape.com/viewarticle/464467_3
Abscess formation. Abscess formation at the site of an IM injection was the first recorded complication of the procedure.[2] It was the most common complication of IM injections in one study, occurring in 31 percent of patients who developed complications.[3] Infectious abscesses following IM injections are caused by the inoculation of the site with bacteria from the needle, syringe, or the medication. The bacteria can be carried to the tissues because of poor site preparation. Inadequately sterilized equipment and medications also can be responsible for inoculation of the area. The majority of these complications present with red, hot masses surrounding the previous injection sites. Occasionally, an abscess will rupture, and the site will be draining pus and liquid fat.[4] Incision and drainage of the area will result in marked improvement in the discomfort and will allow for cultures to be obtained to direct antibiotic therapy. The majority of these cases are seen within a few days to a few weeks following the injection;[4] however, in some cases, an abscess clinically may not be apparent for years after the injection as illustrated by Case 3 of this article. A high index of suspicion must be maintained for uncommon infectious problems after an injection, especially in the immunocompromised patient. For example, there is a report of a leukemia patient with no history of infectious problems who developed a localized mucormycosis infection at the site of an IM corticosteroid injection.[17]
More commonly, the abscesses that are seen at IM injection sites are sterile abscesses. These are nodules of liquefied fat and muscle resulting from necrosis of the involved tissues. Their development has been blamed on a hypersensitivity to the injected medication,[18] but more detailed research has shown that the problem develops when a caustic medication is injected in an inappropriate location.[19] When the medication is injected into the subcutaneous tissues rather than the muscle, absorption is delayed, which allows for a greater tissue reaction to the medication.[19] This reaction is manifested by local tissue necrosis and liquefaction with a surrounding area of intense inflammation. Thus, a painful nodule filled with sterile, liquefied tissue remains at the site. Many times this problem is caused by not using a needle of sufficient length to reach the muscle.[19]
Necrosis. Necrosis of the surrounding tissue following IM installation of a medication was not thought to occur unless the patient was allergic to the medication.[1] It has been found that necrosis of the muscle will occur after any IM injection no matter what medication is injected.[4] The only variable is the size of the necrotic lesion and the severity of it. Forceful placement of a volume of fluid into a closed space will cause damage. In other words, the surrounding muscle and tissues in the immediate area of the needle tip are subjected to the pressure of the mass of fluid that has been instilled into the area, which causes pressure necrosis. The toxicity of the medication, the volume injected, and even the speed at which the injection is given also will influence the size of the necrotic lesion.[20]
Numerous medications have gained notoriety for causing complications at IM injection sites. The significance and magnitude of the damage can be predicted by measuring the serum creatine phosphokinase activity following the injection.[21,22] Cephalothin sodium and tetracycline hydrochloride were the most frequent offenders in one study,[4] but the long-acting injectable medications have recently become the prime offenders.[5] Despite these reports it has been stated that the incidence of complications after administration of any specific agent is generally related to a medication's popularity.[4]