There are multiple things that you need to be wary of. Some men are more sensitive to testosterone than others and more prone to side effects. I'm one of the lucky guys who's never experienced any side effects whatsoever.
Gynecomastia is one of the big side effects you have to worry about. Ever seen the movie
Fight Club? At the beginning of the movie Edward Norton is in a support group with a man who developed man-boobs via steroid abuse. Don't forget; synthetic testosterone
is a steroid and when you mess with your body's hormones you're putting yourself at risk if you don't know what you're doing. Research is your friend. To prevent gynecomastia it's always wise to take an anti-estrogen, such as nolvadex, while you're injecting your testosterone. You'll need an anti-estrogen anyway if you're running a "cycle" where you use testosterone for three or four months and then take a few months off. A lot of people fail to realize that when you inject testosterone into your body, your body actually stops producing its own natural testosterone. Why would it keep producing something that's already there? It doesn't and simply shuts down completely. Hence, when you stop injecting testosterone it takes your body a few weeks (sometimes a few months) to begin producing its own natural testosterone again. Men produce estrogen (just not as much as females, obviously) so during this refractory period where your body isn't producing testosterone, the last thing you want is an overflow of estrogen being released into your system. It'll make you moody, weak, and destroy the gains you've made with the testosterone. Not to mention you won't be able to get a hard-on very easily. This is where taking an estrogen-suppressant comes in handy.
If you take testosterone for too long you run the risk of becoming what bodybuilders and weightlifters refer to as a "lifer." Meaning, your body will never remember how to produce its own natural testosterone and you'll need to take testosterone injections for the rest of your life. I guarantee you guys like Schwarzenegger are still injecting to this very day. That's why most athletes "cycle" their injections by using the testosterone for a few months and then taking a break to let their body produce natural testosterone again.
Another thing you need to worry about is muscle abscesses, which are rare but a major pain that often require surgery if not treated early enough. They're caused by dirty needles, testosterone that's not pure, shooting in the same area too often, or a number of other reasons. It's a muscle infection that hurts like a bitch and requires antibiotics or surgery. If you're going to be injecting testosterone, make sure you know where to do it and rotate those areas regularly. You can't just inject in the same spot over and over again. You need six to eight spots that you cycle through.
Unless prescribed, testosterone is also highly illegal and you'll be facing some serious trouble if you're caught. If you're ordering online, make damn sure the place you're ordering from is legit. When ordering online the testosterone is mailed to you and, as we all know, mailing an illegal substance is a felony. This means if your order is caught in transit you're going bye-bye for a while. Postal workers are federal agents and they treat these things seriously. It's rare, but people do occasionally get busted trying to order testosterone through the mail. The postal worker intercepts the package, informs law enforcement, and law enforcement shows up dressed as a postal worker to deliver your package and have you sign for it. The moment you sign you take a lovely trip downtown.
For the most part I've found testosterone to be fairly harmless. I've had two small muscle abscesses since 2005 that were treated with antibiotics and cleared up quickly. I've had one overseas order seized by customs and a letter mailed to me informing me so, but thankfully no cops came knocking at my door. I've been lucky in that I've never had any real side effects, either.
Everyone's different. You might take testosterone for a few weeks and develop crazy acne or start growing boobs. Or, you might be one of the lucky guys who's just fine. Either way, it's becoming increasingly harder to even get testosterone without a prescription these days. The government's cracking down hard on the stuff thanks to all of these sports scandals. It's harder to score testosterone than it is crack cocaine which is infinitely more harmful. Sad, but true. Getting a prescription is the safest way but, honestly, most doctors don't prescribe a high enough dosage to make much of a difference. I guess it all depends on your goals, really. If you're just an aging male who wants an added spring in their step, a doctor's probably the way to go. However, if you're a lifter who wants to pack on some more muscle, a prescription usually won't do the trick.