Quote Originally Posted by ServerError View Post
If mirtazapine is not a true antidepressant, why is it described everywhere as an antidepressant? And why is prescribed to treat depression?
Marketing. There is no fixed criterion for what can be called an antidepressant (or what can be a SSRI, or SNRI). If a pharmaceutical company wants to call their med an antidepressant it can even if the data says otherwise. Doxepin is another sedating antihistamine which has long been promoted as an AD despite having poor outcomes.

True ADs work by stimulating the growth of new brain cells in the hippocampal regions of the brain. There is no evidence that either mirtazapine, or doxepin do this, afaik.

Another marker for AD effectiveness is reuptake inhibition of one or more of the neurotransmitters serotonin, noradrenaline (aka norepinephrine) and dopamine. Neither mirtazapine or doxepin do this.

Mirtazapine is the second of two 'antidepressants' to be marketed by Organon Pharmaceuticals. The first was mianserin which causes a brief flurry of positive reviews in the medical literature and then sales dropped through the floor as psychiatrists realised the hype didn't match the outcomes they were seeing in patients. Mirtazapine was a rework of mianserin with Organon claiming it has a greater effect on receptors and transporters than mianserin. However, mirtazapine is in fact less potent (see below).

As a guide, SSRIs have a binding affinity of <=2 Ki to the serotonin reuptake transporter molecules (SERT) with paroxetine being the most potent SSRI at 0.08 Ki. The most potent noradrenaline reuptake inhibitor, desipramine, has a binding affinity of 0.83 Ki on noradrenaline transporters (NAT) (Ki = the concentration of the drug needed to block 50% of the target receptors, or transporters, so the lower the value the more potent the inhibition).

Mirtazapine has affinity values of SERT: 10,000 Ki, NAT: 4,600 Ki , but 0.14 Ki on histamine H1 receptors which makes it a more potent antihistamine than meds sold as antihistamines (as examples, desloratadine, is 0.4 Ki and fexofenadine 10.0 Ki)!

The values for mianserin are SERT: 4,000 Ki, NAT: 71 Ki and H1: 1.0 Ki

Doxepin is more potent, SERT: 68 Ki, NAT: 29.5 Ki, but it is a slightly less effective H1 blocker at 0.24 Ki.

Do you have an opinion on its effectiveness in treating depression and anxiety?
I don't rate it for depression, though occasionally it will pull the very depressed, bordering on catatonic back from the brink for a while and its sedating properties can be all some with anxiety need. However, I don't rate it as a first, or second line med for either anxiety, or depression. That said, some do well on it so...

I suggest you have a scroll through some of the posts in this sub forum to get a broader picture of others' experiences.