Hypnotic and sedative agents can become lethal when taken in combination with C. CNS DEPRESSANT AGENTS.
This is what I think.
Hypnotic and sedative agents are suppressants or relaxants. If you combine this with the cns depressants agents, which is also a suppressant, then it would be like having an overdose. As we all know, overdosing on drugs will lead to lethal complications and ultimately death.