system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can a side affects of antidepressants be thoughts of suicide?
gpt
The side effect isn't as direct as your post title makes it seem.

Rather it goes like this:

A heavily depressed person has suicidal thoughts/tendencies, however the depression is so deep that it negates the will/drive to do anything, including committing suicide.

On starting the therapy, the therapist realizes that any treatment will fail due to the absolute lack of drive and deep seated melancholia. He prescribes antidepressants. They work as intended and increase the depressive persons activity level, thus making them theoretically "fit" for treatment. However in doing so, the pharmaceuticals also increased the likelihood of suicide as the treated person now has enough drive/motivation to act, including potentially acting on their suicidal tendencies.

TL:DR
Sometimes the only thing keeping a depressed person from committing suicide, is their total lack of drive to act. Antidepressants may work just well enough to up the drive but not well enough to remove the suicidal tendencies.