Ketamine has a lot of mechanisms of action, but one of its big ones is that it's a major NMDA receptor antagonist. This prevents glutamate from affecting cells

Ketamine has a lot of mechanisms of action, but one of its big ones is that it's a major NMDA receptor antagonist. This prevents glutamate from affecting cells very well. A major decrease in glutamic activity may reduce depression, but it can effectively drop your IQ by quite a lot. People generally report feeling "very stupid" and "brain foggy" after taking strong NMDA antagonists. I don't know if ketamine has high risk of causing general intelligence reduction in the long term, but there are reports of that from people who abuse the drug in recreational doses for a long period of time. So, it's definitely something to be wary about.