In Brief: Do Testosterone Supplements Induce Prostate Cancer?
There is no concrete evidence if testosterone supplements can induce prostate cancer. Some longitudinal studies have shown that higher serum testosterone levels may cause an increased risk of developing prostate cancer, while some other research studies contradict this connection. Therefore, more well-designed research studies are required in the future to eliminate the risks associated with testosterone supplements.
In recent years, men with low testosterone levels are using testosterone supplements to increase the levels of testosterone and thereby have improvements in mood, sexual function, bone mineral density and muscle strength.
As a result, there is a marked increase in the popularity of these supplements.
Accompanying the use of testosterone supplements is the concern of potential adverse effects, including the development of prostate cancer and cardiovascular disorders.
Some studies indicate that higher levels of free serum testosterone may increase the risk of developing prostate cancer and promote cancer progression. 
Huggins and Hodges demonstrated in 1941 that the administration of testosterone promoted the growth of prostate cancer and that reducing testosterone levels caused the regression of metastatic prostate cancer.
In a Baltimore Longitudinal Study of Aging comprising of 781 men, the risk of prostate cancer doubled per unit increase in the free testosterone index in men over 65 years.
In another study, testosterone replacement therapy, when given intramuscularly, caused a minimal increase in prostate-specific antigen levels, which are a primary screening tool for prostate cancer.
However, a meta-analysis review performed using random-effects models showed that the risk of prostate cancer did not relate to endogenous testosterone levels.
Another study conducted in hypogonadal men showed that testosterone replacement therapy did not increase prostate-specific antigen levels.
A Korean study showed that patients with low levels of serum testosterone had a significantly high prostate cancer incidence than patients with elevated serum testosterone.
Also, one study showed that testosterone replacement therapy in selective prostate cancer patients after brachytherapy did not lead to the progression of prostate cancer.
A collaborative analysis from 18 prospective studies involving 3886 men with prostate cancer reported that the serum concentrations of sex hormones were not associated with the risk of developing prostate cancer.
Therefore, currently, there is no clear evidence of whether testosterone supplements can fuel prostate cancer in humans. There is growing evidence from small randomized trials that testosterone supplements are potentially safer.
However, more extensive randomized controlled trials with more number of patients and men with a history of prostate cancer are required to monitor the potential risks associated with testosterone supplements in the future.
Also, it is safer to consult a doctor before taking these supplements if having a history of prostate cancer.