Aromatase Inhibitors (AI) help prevent the conversion of testosterone to estrogen. As you would expect, as T levels rise, more E is subsequently produced. We must give some thought to the timing of the AI compared to the administration of the TRT. And the form of TRT you use. For Arimidex (anastrazole), just figure it's good for two days. If you are using a T gel, more consistent administration of the AI is necessary, because you must apply the gel every day. For shots, it's a different story. When do you feel the estrogen, compared to when you take your shot? The T shot peaks in about 48 hours. That would then be when you want the AI doing its best work as well. OTOH, there is something about rising estrogen levels which bothers many men, so they need to take their AI at the same time as their shot. As always, every-body is different. Even though the anastrazole is pretty much worn off by the third day, some guys only have E issues right after their shots. So they may take the AI sometime around their shot (same day, day after, whatever they have found works best for them). So it may be only once per week. For those on twice per week split dose regimens (which tends to lower E anyway), they might take their pill each time they have a shot. Then there is the interplay of the SHBG in all this, as always. More on that later. Just knowing what your options are is a big help; this is why the assistance of a properly trained medical professional is necessary for so many.