Prozac is perhaps the most well known of the many different medications prescribed to help people whose symptoms of depression are caused by an imbalance of chemicals in the brain. Marijuana, in its own drug class, is not a strict depressant. It has different effects based on the user's mood, surroundings, dose, and frequency of use. These can range from sedative to hallucinogenic to stimulant. Interestingly, marijuana and Prozac can actually produce many of the same emotional effects.
Some people experience insomnia while taking it.
It's possible that the changes you have noticed in your friends are these. On the other hand, marijuana is also known to cause mood swings, sleepiness or sluggishness, and, sometimes, decreased motivation. In higher doses, hallucinogenic effects are possible, which may be expressed through anxiety or rapid changes in mood. Sometimes marijuana is laced with other drugs, like LSD or PCP, and then misperceptions, or even a sense of paranoia, are even more likely. If they stopped or lightened up a bit on their use, they might find that they felt better, perhaps even without the anti-depressant.