It didn't take me that long (actually only 3 clicks) but I did find mention that it can rarely cause drowsiness.
From:
N-ACETYL CYSTEINE Side Effects & Safety
Quote:
N-acetyl cysteine is LIKELY SAFE for most adults, when used as a prescription medication. It can cause nausea, vomiting, and diarrhea or constipation. Rarely, it can cause rashes, fever, headache, drowsiness, low blood pressure, and liver problems.
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emphasis mine.
I don't know why either. Take at bedtime? (And rejoice you haven't suffered those other side effects...

)
The smallest capsules I've seen are 500 mg, but I've seen it sold in bulk too, so one could measure out smaller amounts.
Doc