Unfortunately, the same study that tells us that MENT is 10x as anabolic as testosterone, and 4x as androgenic, also tells us that MENT is 12x as suppressive to serum gonadatropins as testosterone. For this reason, it is very appealing to scientists who have studied it for use as a male contraceptive – and ended up calling it “ The Optimal Androgen for Male Contraception ” (Ann Med. 1993 Apr;25(2):199-205.). In addition, and partly owing to its inability to be 5a-reduced, MENT is converted (via the aromatase enzyme) to a very potent form of estrogen – which can cause gynecomastia (enlarged breast tissue in males), and several other nasty side effects.
Heavy consumption of the essential amino acid lysine (as indicated in the treatment of cold sores) has allegedly shown false positives in some and was cited by American shotputter C. J. Hunter as the reason for his positive test, though in 2004 he admitted to a federal grand jury that he had injected nandrolone.  A possible cause of incorrect urine test results is the presence of metabolites from other AAS, though modern urinalysis can usually determine the exact steroid used by analyzing the ratio of the two remaining nandrolone metabolites. As a result of the numerous overturned verdicts, the testing procedure was reviewed by UK Sport . On October 5, 2007, three-time Olympic gold medalist for track and field Marion Jones admitted to use of the drug, and was sentenced to six months in jail for lying to a federal grand jury in 2000. 
I do not consider trenbolone to be capable of causing or aggravating gynecomastia. I do not know of a single case where the source of trenbolone was Parabolan (back when it was available), Finaplix H, or Component T-H. The problem appears to occur only when the source is an underground lab or the home preparation is made from powder from a black-market supplier. Since trenbolone’s wholesale price direct from a manufacturer is much higher than that of most anabolic steroids , it shouldn’t be surprising that substitution or partial substitutiion would often occur with underground products. Additionally, it might be that some have mistakenly made preparations from Finaplix S, which definitely would be capable of causing gyno, as that product includes estradiol benzoate as an ingredient.