http://boards.elitefitness.com/forum/showthread.php?threadid=18192
Please read carefully the whole thread. Especailly, post by Anabolicum Mister. Even, theoretically, possibility of 5-alpha reduction was esteblished, but, there's only one, shady looking study from China, which said that
"possibly" 5-alpha metabolite of Trenbolone was found.
On the other hand, there are a lot of studies that don't show presence of that metabolite.
So, either China study are not sure what they found, otherwise why would they use word "possible", or it's pure speculation.
And, even if it's a fact, then, still, like AM posted, the affinity of 5-AR enzyme to Tren has to be VERY weak.
Conclusion: you can use Finasteride with Tren, if you want, but it's not going to do any benefit, neither harm, in sense of hairloss.
Still, it is important to know for the ones who stack Tren with Test.
Personally, I'm on Tren for 4 moths now, and notice some increase in hairloss (I'm prone to MPB), despite Propecia, Saw Palmetto, Xandrox and Nizoral. But, I think, it's due to Tren being strong androgen itself.