If this happens most people believe there injects are contaminated with bacteria. That is of course a possibility but it can also have a pyrogen (endotoxin) inside.
But sometimes the body releases a (endogeneous) pyrogen. An allergic reaction is also possible just like a allergic reaction on one of the solvents, perservatives and..acids or estrifications.
The Danish student Michael Timm did some research on pyrogens "in vitro" on AA steroids, we will cover it with an article. Especially prone to pyrogen release are Nor-19 steroids such as Boldenone and Nandrolone and then especially the short estrifications.
Studies on steroid fever II. Pyrogenic and anti-pyrogenic activity in vitro of some endogenous steroids of man
G. M. Dillard and Phyllis Bodel Department of Internal Medicine, Yale University School of Medicine, New Haven, Connecticut 06510 Published December, 1970
The pyrogenic properties of some C-19 and C-21 steroids were examined by in vitro incubation of human blood leukocytes with serum-buffer solutions of the steroids and injection of the 18-hr supernatants into rabbits. In previous studies this method demonstrated release of leukocyte endogenous pyrogen by etiocholanolone. With two exceptions, steroids known to cause fever in man, such as 11β-OH etiocholanolone and 3α-hydroxy-5β-pregnane-20-one were also pyrogenic in vitro. All steroids tested which are nonpyrogenic in man, such as androsterone, 3β-OH etiocholanolone, and 3α, 17α-dihydroxy-5β-pregnan-20-one were also nonpyrogenic in vitro. Solubility in aqueous solution did not correlate with pyrogenic capacity.
Inhibition of pyrogen release from human leukocytes in vitro by hydrocortisone and estradiol was demonstrated. Hydrocortisone-treated leukocytes released less pyrogen than did normal leukocytes when stimulated either by etiocholanolone or by phagocytosis of heat-killed staphylococci. On the other hand, estradiol-treated blood leukocytes and mononuclear cells showed significant suppression of pyrogen release when phagocytosis, but not etiocholanolone, was used as the stimulus. When blood cells were incubated with progesterone, greater than normal amounts of pyrogen were released following phagocytosis, and the inhibiting effect of estradiol could be partially reversed. Neither estradiol nor hydrocortisone appeared to act on rabbit leukocytes.
These studies indicate that a variety of naturally-occurring steroids may alter pyrogen release from leukocytes. Alterations in steroid balance in man may influence normal temperature regulation and contribute to clinical fevers.