Contrary to another post here, SSRIs in my opinion and an increasing majority of researchers studying this field believe that SSRIs and the so called 'antidepressant' drugs cause great harm. SSRIS which are actually
Firstly here is a list of common side effects: Insomnia (which is common in depression anyway), increased anxiousness, feeling sick, indigestion, diarrhoea or constipation, loss of appetite, fluctuations in weight, dizziness, blurred vision, dry mouth, excessive sweating, headaches, reduced sex drive, difficulty achieving orgasm, erectile dysfunction.
None of these are good effects which is why we don't give them to people who are 'healthy'. More importantly the devastating effect these side effects can have on a patients life could potentially be far more harmful than before the drug was taken, for example increased marital problems. According to Joanna Moncrieff these effects compound depression, and more importantly desisturb the chemical balance in the the brain, and as a result compound or cause the very symptons they are meant to cure. Central to this notion and the question presented here, the long-term use of SSRIs may causes what we call 'serotonin induced apathy', in other words, after initial positive effect of SSRIs (quiet possibly short-term placebo effect), patients become indifferent to their environments. Furthermore Long-term use of SSRIs leads to abnormal brain neurogenesis not seen in non-depressed samples, this is often taken to as proof of how SSRIs exert there action, or rather it was before 2008 (see below meta analysis). However it seems that this neurogenesis is less likely to be positive considering the negative effects and lack of efficacy SSRIs display.
The long-term use of SSRIs often leads to the development of apathy, which utimatley will lead many patients back into depression. The rate of relapse in people taking 'anti-depressants' is >40% while patients with placebo is roughly 24%. If that isn't bad enough at a 12 month follow up only <5% of patients are in remission for their depression. Why might this be? Well a 2008 meta-analysis by Kirsch et al found that all 'anti-depressants' are below clinical levels of acceptance, in other words no more effective than placebo. However it should be noted that in serve depression SSRIs are useful only!
The reason anti-depressants such as SSRIs are ineffective is because of the simplistic approach taken to treatment with these drugs. The brain is not some 'big bag of chemicals', neurons are not purely serotonergic in fact they often release multiple NTs. And crucially serotonin NT is used throughout the central nervous system, for more than mood regulation. The fact that SSRIs do not work over the long-term tells us the serotonin theory of depression is wrong. Perhaps more importantly roughly 10% of people in the UK are on 'anti-depressants', we can talk about over medicalisation of mental health, but the reality is large amounts of people are taking these drugs for large amounts of time (9-12 months recommend). This is not helped by the fact that governments are more interested in investing in cheap medical 'cures' than long-term methods with greater efficacy that cost more.
Sorry its so long, and I'm sorry its so short there is so much to say about how SSRIs don't work and how bad they are regarding people thinking they are getting effective cures, along with the neuroscience and discussions on publication bias. Its a fascinating topic, thanks for the question.