Really? I'll have to keep that in mind for next time.
Both compounds are structurally alike, classified as triphenylethylenes. Nolvadex is clearly the stronger component of the two as it can achieve better results in decreasing overall estrogen with 20-40 mg a day, than Clomid can in doses of 100-150 mg a day. A noteworthy difference. Triphenylethylenes are very mild estrogens that do not exert a lot, if any activity at the estrogen receptor, but are still highly attracted to it. As such they will occupy the receptor and keep it from binding estrogens. This means they do not actively work to reduce estrogen in the body like Proviron, Viratase or arimidex would (by competing for the aromatase enzyme), but that it blocks the receptor so that any estrogen in the body is basically inert, because it has no receptor to bind to.
This has advantages and disadvantages. The disadvantage is that when use is discontinued, the estrogen level is still the same and new problems will develop much sooner. The advantage is that it works much faster and has results sooner than with an aromatase blocker like Proviron or arimidex. Therefor, when problems such as gynocomastia occur during a cycle of steroids one will usually start 20 mg/day of nolva or 100 mg/day of Clomid straight away, in conjunction with some Proviron or arimidex. The proviron or arimidex will actively reduce estrogen while the Clomid or Nolvadex will solve your ongoing problem straight away. This way, when use is discontinued there is no immediate rebound.
So which one should you use? Well personally, I'd have to say Nolvadex. Both as an on-cycle anti-estrogen and a post-cycle therapy. As an anti-estrogen its simply much stronger, demonstrated by the fact that better results are obtained with 20-40 mg than with 100-150 mg of Clomid. For post-cycle, this plays a key role as well. It deactivates rebound estrogen much faster and more effective. But most importantly, Nolvadex has a direct influence on bringing back natural testosterone, where as Clomid may actually have a slight negative influence. The reason being that tamoxifen (as in Nolvadex) seems to increase the responsiveness of LH (luteinizing hormone) to GnRH (gonadtropin releasing hormone), whereas Clomid seems to decrease the responsiveness a bit1.
Another noteworthy fact about Nolvadex is that it acts more potently as an estrogen in the liver. As you remember, I mentioned that clomiphene and tamoxifen are basically weak estrogens. Well, tamoxifen is apparently still quite potent in the liver. This offers us the positive benefits of this hormone in the liver, while avoiding its negative effects elsewhere in the body. As such Nolvadex can have a very positive impact on negative cholesterol levels2 in the body, and therefore too should be considered a better choice than Clomid. It will not solve the problem of bad cholesterol levels during Steroid use, but will help to contain the problem to a larger degree.
Another reason why I promote the use of Nolvadex over Clomid post-cycle (as if being 3-4 times stronger and having more of a direct effect on restoring natural test wasn't enough) is because it's a lot safer. Not just because it improves lipid profiles, but also because it simply doesn't have the intrinsic side-effects that Clomid has. Clomid causes more acne for sure, but that's mainly because you need to use a 3-4 times higher dose. But Clomid seems to also affect the eyesight. Long-term Clomid therapy causes irreversible changes in eyesight3 in users. Irreversible. For me that alone is reason enough to prefer Nolvadex.
Lastly, one should be aware that use of these compounds can reduce the gains made on steroids. Nolvadex more so than Clomid, simply because it is stronger. Estrogen is responsible for a number of anabolic factors such as increasing growth hormone output, upgrading the androgen receptor and improving glucose utilization. This is why aromatizing steroids like testosterone are still best suited for maximum muscle gain. When reducing the estrogen levels, we therefore reduce the potential gains being made. For this reason one may opt to try Clomid during a cycle instead of Nolvadex. Although I would imagine that the problem that needed solved would be of more concern, in which case nolva remains the weapon of choice. It's a plain fact that there is a high correlation between gains and side-effects. Either you go for maximum gains and tolerate the side-effects, or you reduce the side-effects, and with it the gains. That's life, nothing is free.
