ok, the following is simply "my opinion". it is based on a good bit of AAS knowledge, and some past experience as well. This is by no means gospel, it is simply "my opinion" on why a guy should always include test in his cycle:
Though it is true that all AAS are in some way, shape, or form based on testosterone, all of them are chemically "different" than testosterone. In other words, they are more "foreign" to the body. Esterfied testosterone (or any synthetic testosterone) is a foreign substance to the body, this is true, however the body is more "used to" testosterone than say nandrolone for example. This is often times the reason testosterone is recommended as a first cycle.
This is one reason Test is often recommended as part of every cycle. Usually new users are advised to add only one new substance at a time. For example:
1st cycle Test E
2nd Cycle Test E and DBol
3rd Cycle Test E, DBol, and EQ
4th cycle TEst E, DBol, EQ, and Deca
etc, etc.
of course some people will never use more than 2 drugs, or some only one, because everyone is different.
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another thing to consider is something that has been touched on earlier in this thread: comfort.
As men, our bodies get accustomed to having testosterone in our bodies. When "cut our bodies off" from testosterone, it often makes us feel differently. Some guys get depressed, some feel that they have no energy, some stay angry and frustrated, etc. Of course there is also sexual functionality that is involved, that may or may not be affected by other drugs.
Keep in mind that if test makes you feel more "normal", then the side effects you experience from other drugs will be less impactful. If you are already pissed off because of a lack of testosterone, I can imagine that the increased aggression from tren or halotestin, can only be more likely to be "misdirected".
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the third and final one I will address (a.k.a. that I can think of off the top of my head), is cycle length. Which actually the other two tie into this.
Now, I'm a young buck, but from what I've read about "old school" cycles, they usually weren't very long. 8-10 weeks or so. If I'm wrong, then the rest of what I'm about to say is pure BS, but from my understanding this is the case.
Today, long cycles are not uncommon. some guys stay on year-round even. I could not see staying on year round, or even a 16-20 week cycle without testosterone. That would be horrible for me personally, and I think a lot of people feel that way.
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once again, this is just my opinion, but I think it's one that a lot of people share. Hopefully this is along the lines of the kind of answer you were looking for. If you want me to explain anything in more detail, feel free to post, or PM me. Or if you just want to comment, go for it.