Here ya go bro.. I'll do some follow up articles if I find any other formal studies... it looks like it is caused by the actual transport vehicle for the compounds at least based off of this study...
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From the Oxford Journals - Human Reproduction
Andrology: Tolerability of intramuscular injections of testosterone ester in oil vehicle
Mary-Anne Mackey, Ann J. Conway and David J. Handelsman1
Andrology Unit, Royal Prince Alfred Hospital, and Department of Medicine and Department of Obstetrics and Gynecology, University of Sydney Sydney NSW 2006 Australia
Correspondence: 1To whom correspondence should be addressed at: Andrology Unit, Suite 312C, RPA Medical Centre, 100 Carillon Avenue, Newtown NSW 2042, Australia
We undertook a prospective survey of the tolerability of deep i.m. injections of testosterone enanthate in a castor oil vehicle, the most widely used form of androgen replacement therapy. Over a period of 8 months, 26 men received 551 weekly injections into the gluteal, deltoid or thigh muscle and side-effects were recorded immediately and 1 week after each injection by the same nurse using a standardized questionnaire. Most injections caused no complaints [389/ 551, 70.6% (95% confidence interval 66.6–74.4%)] but minor local side-effects, mostly pain and bleeding, were common [162/551, 29.4% (25.6–33.4%)]; no serious side-effects were observed. Considering all side-effects, the gluteal site had fewer complaints and was less prone to bleeding but was painful more often than deltoid or thigh injection sites. The laterality of injection at any site had no significant effect on side-effects. The only systemic side-effect was episodes of sudden-onset, non-productive cough associated with faintness following eight injections [1.5% (0.6–2.9%)] which we speculate may have been due to pulmonary oil microembolism. We conclude that, when administered by an experienced nurse, deep i.m. injection of testosterone enanthate in a castor oil vehicle is generally safe and well tolerated but causes relatively frequent minor side-effects, including pain and bleeding. An improved depot form of testosterone would be highly desirable for androgen replacement therapy and hormonal male contraception.
