With any injected formulation damage occurs to the surrounding tissues. In the case of an intravenous injection, this usually has little consequence for drug absorption, but in the case of intramuscular and subcutaneous systems the drug or device is inherently present in a wound site and the body will react accordingly. The reaction depends on the size and composition of the device. Small isolated particles less than 10 pm will be engulfed by macrophages without any major reaction occurring, but larger objects in a microsphere form will gather a layer of macrophages and giant cells adhering to the surface of the particles. Larger devices with large surface areas will elicit a foreign body reaction which begins with inflammation and the formation of a layer of macrophages and giant cells, and is followed by the formation of granulation tissue. Ultimately a fibroid capsule consisting of fibroblasts and collagen will surround the device. The consequences of these changes for drug delivery have not been widely explored but at present it is thought that they make little difference to drug release and absorption unless the reaction is severe.
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