The kidneys are under more strain during steroid intake. They are involved in the filtration and excretion of toxic by-products. A high blood pressure as well as variations in the water and electrolyte balance of the body can lead to long-term changes in the kidney's function. A Wilm's tumor, a fast-growing kidney tumor normally only seen in infants and children has been noticed in certain rare cases in athletes using steroids. It is doubtful if there is a direct connection between the two. It is certain though, that during steroid consumption several athletes may develop a dark-colored urine and, in extreme cases, even blood in the urine. The former Finaject and today's Parabolan, in particular, seem to have a toxic effect on the kidney function.
The occurrence of side effects is different from one athlete to another. Factors such as age, gender, constitution, the respective physical and psychic condition of the individual, as well as the dosage, the length of intake, and the selection of the steroid play an important part in the development and seriousness of side effects.