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In this thread I will review the main causes of localised muscular pain, tenderness and soreness that are experienced as a result of intramuscular (IM) injections. There are two main causative groups which injection pain can be categorised: 1. Pain due to route of administration 2. Pain due to the substance being injected Within these two categories there are further sub-groups for causes of injection pain, and these will be discussed in further detail. 1. Pain due to route of administration 1. Invasiveness of Injection. The initial cause of injection pain that may be experienced is quite simply the fact that an IM injection itself is an invasive procedure, in that the body's natural barrier of skin is being penetrated by a sharp needle and any further cellular content along the needle's path is being sheared. This in itself, although relatively invasive, can cause some pain. This pain tends to be initial, however due to the design of sterile needles for injection, the curved nature of the needle point allows for minimal pain, and thus this is not a common cause of post-injection pain. 2. Opening a new IM injection site. When a new muscular site is invaded and a volume of substance is injected therein, there is often some soreness associated with such an injection. The muscle group is not used to containing an additional volume of a substance, thus pain can result. This is usually only experienced when a site is first used for an IM injection. This is why when using a new site it is encouraged to inject a smaller amount initially, from 0.5-1ml dependent upon muscle size. Such pain and soreness usually ceases after a site has received at least one injection. 3. Physical location of injection. Often when injecting a substance, the physical location, that is where the needle releases the substance, can result in discomfort after the injection. This is most likely due to the substance (especially if oil based and slowly absorbed) sitting in between muscle groups or in a small muscle, as this will cause more pain than being injected into the middle of a muscle or a larger muscle. This can happen from time to time even with experienced users. 4. Volume of substance injected. Tying in with point 1(ii), the volume of injection will also make a significant difference to any soreness and pain experienced. Generally larger volumes are better tolerated in larger muscle groups (gluteus, quadriceps, etc), with smaller muscle groups (biceps, triceps, etc) fair better with smaller volumes (5-6ml, especially of oil-based substances) are not advised due to the risk of developing a sterile abscess. 2. Pain due to the substance being injected 1. Abscess development. As with any substance, unless it is sterile (and even in sterile cases bacteria on skin and body hair may be pushed past the skin barrier inside the body by the injection procedure) there is a high risk of developing an internal infection known as an abscess. This will result in large amount of swelling, redness, flu-like symptoms and increased lymphocytes thus increased inflammation resulting in a fair degree of pain experienced. The risk of such infections being developed when using completely sterile products however is very low. 2. Solvent concentration of substance. The concentration and type of solvents used in the preparation of the substance to be injected will affect any pain and soreness that will be experienced post-injection. Certain buffers and solvents used when injected in pure form give a fair amount of soreness. For example, some peptides are suspended in 100mM acetic acid, which when injected even in small amounts can give the user some discomfort. It is an important point to make that pain resulting from solvent concentrations used is most likely to commence quite soon after the injection, from a few minutes to a few hours maximum. Pain that takes longer than this to develop is usually due to other factors discussed here. Another common solvent used as a preservative in anabolic steroid and other IM injection preparations, is benzyl alcohol (BA). At high concentrations, BA will cause injection pain in the vast majority of users. However in concentrations