pibbur who has first hand experience with several of the drugs and are quite familiar with some of the side effects.
I am not familiar with anti-depressant medication nor with depression myself, but I do know that depression is a symptom of deficiency of vitamine B12 (methylB12) and/or methylfolate. So supplementing hydroxyB12 or methylB12 and methylfolate can alleviate or even cure depression in some cases.
Some people have problems absorbing B12, they need injections or tablets (sublingual). B12 is a vitamin that can be found in animal products only, so veganists (and some vegetarians) will get into trouble too if they are not supplementing B12.
Folinic acid is present in dark green vegetables, the body turns it into methylfolate, but some are not able to produce enough methylfolate.
B12 and methylfolate are essential for normal functioning of body and mind. B12 for instance is needed for the insulation surrounding the nerves (myelin). Hence a lack of B12 may lead to very different symptoms like depression, apathy, mood swings, brain fogs, concentration and memory problems, dementia, aphasia, numbness or tingling or prickling sensations in arms, legs, hands or feet, coordination problems, imbalance, anomalous reflexes, shortness of breath, muscle weakness, extreme fatigue, feeling cold, weight loss, etc. etc.
It is important to have enough B12 when supplementing methylfolate or folic acid.
If you'd like to learn more, here are some keywords: methylation, methylcobalamin, adenosylcobalamin, Metafolin, Deplin.
An interesting documentary:
Diagnosing and treating vitamin B12 deficiency
Studies have shown vitamin D deficiency and depression are also related btw. But I am not familiar with the specifics of vitamin D deficiency.