The linked tumblr post explains in detail a dangerous scam going around twitter's trans community.

A product called "I can't believe it's not estrogen" is being advertised as a natural replacement for estrogen. It is packaged in a trendy, eye-catching way and it is sold as an OTC supplement for people who can't get access to actual HRT.

In reality, it is at best a scam that will never ship and it aims at collecting trans women's personal details in order to harass them later on. At worst, it is a scam that directly aims at physically harming trans women. The substance contained in the advertised pills is ashwagandha, which boosts the production of the hormones your body produces on its own (i.e it would boost T production for a trans woman). Furthermore, the dosage found in these pills increases anxiety levels. If taken for several weeks in a a row, it will cause something called "serotonin syndrome", which will send you to the ER, potentially at death's door.

If you are in queer twitter circles, spread the warning around to keep our community safe.