Having read this thread I had a VERY slow sink. I poured in Draino gel and saw it contained sodium hydroxide and sodium hypochlorite as its active agreements. The stuff opened up 2 slow drains but one was still no better.

So I went out to the pool, got a cup or two of 12.5% from the carboy and poured THAT down the sink, waited a half hour and VOILA! fast-running sink again! That was yesterday.