Studhammer has hit it on the head, deca is hormone roulette. I have always just been to scared to try it. I finally felt comfortable taking NPP, same hormone as deca but shorter ester so shorter half life. Shorter half life means it clears the system quicker. I didn’t have any issues on NPP. Somehow all the issues seem to happen with deca not it’s shorter chain sister NPP.
I would recommend the NPP route for everyone, it just gives you a way to try the hormone and see if your dick still works. If it starts not working then the NPP stops being able to affect anything in a much shorter time frame. The same mentality is used with tren, use tren a first to see if you can handle the whole tren experience.
Since this is your first stack I would recommend you switching to EQ. They use to say EQ and deca are similar but really they are not. EQ is truly a dual purpose compound build or cut with it. It has a similar ester to deca so it needs to be a cycle length of at least 12 weeks. It too aromatizes but it just has Estrogen side effects to deal with (arimidex will cover you). Deca is a progesterone so it has other side effects that arimidex can not address. Both will give you slower steady quality gains over the entire cycle. That means you get to keep most of them as long as you PCT and handle your end properly.
A lot of guys have no issue with deca but those that do curse it, wouldn’t you if all of a sudden your dick didn’t work?
As far as mixing eq or deca into a syringe with test, yes totally fine to do.
With either EQ or deca you can start them with the test and either one I would stop and run test another two weeks after you stop the EQ or deca.
Both EQ and deca have very long esters so that effects PCT timing. You need to wait at least three weeks after your last shot of either. So if you stop either one and continue the test two more weeks then stop you have to wait two weeks for the test cypionate to clear before you start PCT. If you stop EQ or deca at the same time as the test then wait three weeks you are going to have no test in your system, not good. The whole running test another two weeks is VERY important to do with deca, it’s good to do with EQ but with deca it is important. Mainly it helps keep the deca from attaching to the penile tissue receptors, that’s how you get deca dick aka a flabby floppy sexually useless cock.