Most drugs have zero effect if you have ptsd, clinical depression and schizoaffective or similar; LSD will probably make it worse and party drugs (amphetamines) won't work on you. Everyone else will get high, and you'll be bored and sober.
That said, if you are on MAOI's or SSRI's, you would do well to end their use before attempting any hallucinogen stronger than cannabis.
In terms of reactions, I think it varies from person to person. Generally speaking though, hallucinogens aggravate existing psychotic disorders. If you suffer from PTSD/depression it's not a psychotic disorder (so it's relatively safe for you to take).
Curiously enough, a subset of patients suffering from ptsd/depression/anxiety don't experience the euphoria associated with amphetamines. I'm sorry if I didn't word myself clearly enough