BB is basically a solvent, it provides a means of holding the testosterone in suspension at higher concentrations. It has a very low viscosity.
Both BB and Benzyl Alcohol are added. BA added to lower the risk of bacterial contamination.
Both of these can be cytotoxic at higher concentrations.
For my own brews I don’t use any BB, instead using Mig840 for the lower viscosity allowing more hormone to hold in suspension. BB is nasty stuff, put a drop of it on your tongue and you’ll see why I don’t like the stuff going into my body.
BA is just about a must for those sourcing to the public, at 2% it helps lower the chances of bacteria growing in your vials. I would personally recommend heating any vials before first time use on a candle warmer for 45 minutes to insure any microbial life is completely killed due to heat at 145 F.
I brew for myself without BA using small 5ml vials, smaller vials so it’s used quickly in a few weeks, no chance of contamination causing bacterial growth.
Southern Compounding does a good job of creating a brew that doesn’t cause pip. I’ve used a lot of his gear and have lots of it on hand and never had a problem as long as I stick to the cypionate and acetate esters.
BB is standard among just about all sources, it’s not absolutely necessary, but it would force them to brew at lower concentrations, for a homebrewer like myself that’s fine, I just inject more, for the public that’s not gonna work.