服用这种药物后，他开始出现听见不存在的声音的幻觉。

After taking the medication, he started having ___hallucinations of hearing voices that weren't there.

他以为自己在沙漠中看到了绿洲，但那只是幻觉。

He thought he saw an oasis in the desert, but it was just a ___hallucination.