Is tamoxifen anestrogen receptor agonist?

Yes and no. Tamoxifen is a partial estrogen agonist antagonist. Help reducing breast cancer risks, which have a positive effect in increasing bone mineral density.
Yes but weak. Tamoxifen is a very weak estrogen receptor agonist. It is frequently used to help treat breast cancer by binding to the receptors that are normally binding Estradiol (a very potent estrogen receptor agonist). If the tamoxifen gets to the receptor first, then it blocks the estradiol.

Related Questions

Can you tell me in myasthenia gravis why do you give an acetylcholine esterase inhibitor instead of a direct receptor agonist?

Because... If you provide an agonist your muscles will contract continually. The acetylcholine esterase inhibitor prolongs the life of acetylcholine released naturally from your motor nerves so you only have muscle contraction when your brain orders it. Read more...