General Synonyms:
* Risky
* Hazardous
* Perilous
* Threatening
* Unsafe
* Precarious
* Treacherous
* Harmful
* Deadly
* Lethal
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More Specific Synonyms:
* For physical danger: Life-threatening, perilous, precarious, risky, hazardous, harmful, deadly, lethal,
* For emotional or psychological danger: Threatening, toxic, destructive, harmful, destabilizing
* For moral danger: Corrupting, unethical, immoral,
The best synonym will depend on the specific situation you are describing.