Anastrozole

Tamoxifen lowers anastrozole plasma levels by roughly a quarter while the two suppress estrogen signalling by different routes, so the combination is both less predictable and easier to overshoot than either alone.

Estradiol drives prolactin secretion, so an aromatase inhibitor is already lowering prolactin, and adding cabergoline on top suppresses both further while the erectile dysfunction being treated is usually the crashed estradiol itself.

Adding an aromatase inhibitor to a SERM restart removes the estradiol the recovering axis and the lipid panel both depend on, while contributing nothing to the gonadotropin drive clomiphene is there to produce.

Two aromatase inhibitors at once suppress estradiol far past the intended target, and exemestane's irreversible binding makes the resulting crash slow to reverse.

Both push the lipid panel the same way, isotretinoin by raising triglycerides and lowering HDL and anastrozole by removing the estradiol that maintains HDL, so the combined panel is worse than either predicts alone.

Both are reversible non-steroidal aromatase inhibitors competing for the same enzyme, so the pair only compounds estradiol suppression and the lipid and bone cost that follows.