Bull Shark Testosterone is a steroid made from the testicles of Chilean bull sharks. Brucie Kibbutz is a known user of bull shark testosterone, and has even made a company selling it as of 2013. It is mentioned several times throughout the HD Universe, and has even become a usable item in Grand Theft Auto Online.
Bull shark testosterone has many effects on users. They include excessive roid rage, enhanced strength, and higher reflexes. Brucie has been turned into a maniac just from taking too much bull shark testosterone. However, the GTA Online Protagonist seems to experience no long term effects from taking bull shark testosterone. This may be for gameplay reasons.
In GTA Online, the player is able to call Brucie when Rank 17 is reached to have him deliver a box of bull shark testosterone to the player. The bull shark testosterone costs $500. In Deathmatches, there will be bull shark testosterone packages scattered throughout the map.