Refers specifically to testosterone that is bought without a prescription.
Testosterone comes in a number of forms, the most common of which is injectable T.
Street T refers specifically to the hormone as it is used by some ftm guys (aka transmen) as well as some butches, aggressives, genderqueers and gender variant folks as part of a transition process or just because they like what it does.
Side effects can include: voice deepening, acne, genital changes, fat redistribution, body hair growth, facial changes, changes in body odor, blood pressure, and more.
Street T can be dangerous for many reasons including needle sharing and using a pre-used vial of T, but for many is the only available or affordable way to get it.
"I heard that Harry has stopped being able to afford his prescription and started using Street T."