I have been on fluoxetine since it was first avalable in the U.S., over 25 years ago, and have been on many other antidepressants.
Your question cannot be answered.
Tens of millions of people take antidepressants and each person has hundreds of millions of brain cells and neurotransmitters, firing in their own unique way -- that is, no ones brain reacts to any drug as does your brain. On depression forums -- like this website but just mental health posts -- they say "YMMV" -- your mileage may vary -- hundreds of times a day to questions like yours.
Two ideas: Many, if not most people suffering moderate depression, do not get better because they do not stick with the drug long enough and/or are not at high enough dose. Many studies have shown, for example, that most general practitioners, family doctors in the U.S., only prescribe up to 40 mg. of fluoxetine. Usually -- YMMV-- most side effects go away in four or six weeks, unless they are sleep or tiredness related. So fluoxetine can be taken to 80 mg., with only some chance the side effects would get worse. I would bet the drugs you have been on have not been given a chance to work at high enough dose. It takes months, sometimes, building up a drug. Fluoxetine takes a long time to build up, six to eight weeks for some. Effexor, venlafaxine, does best in studies of people with severe depression. It has helped me and people I know. But, again, YMMV. It is tough to build up -- so if you are very depressed -- there often is not the time to wait. And millions of people will tell you it is a bitch to get off.
Second, look at all of the stuff people are given as adjuncts, to boost the antidepressant if you can tolerate more, such as a stimulant, lithium, or what works for me best, and has the least side effects, wellbutrin (bupropion), which also helps some people dealing with fatigue.
People can only tell you their experiences. I cannot emphasize that enough.
The research done on these drugs is very bad and provides no map on what to do, since the drugs often don't do better than nothing, placebo. If you have severely depressed people, such as in hospital, then the drugs will often do 10-20% better than placebo. If you do not like mirtazapine, you have to have a few real trials on fluoxetine, sertraline, venlafaxine, citalopram,etc. -- depending on what they presribe in the U.K.
Then if it doesn't work, or doesn't work fully, depending on what the NHS will give you, try adding an adjunct (another drug). I take Adderall, an amphetamine, while millions of others take ritalin, dexedrine, and other stimulants. Many people find adding thyroid medication helps the antidepressant. There are a bewildering number of combos, but one thing can be said and is true much of the time -- people do not build up slowly, take enough, long enough, and often do not try combos if one drug doesn't work before abandoning that drug.
If your depression is severe, you cannot function, or at risk of harming yourself, ECT is still the most effective treatment, especially unipolar -- just one side of the brain -- not bipolar, both sides of the brain: unipolar has less chance of causing memory or thinking problems, but I do not know what they offer in the U.K.Don
