One might say, "benzodiazepines have a similar effect to alcohol", or "benzodiazepines have an effect similar to alcohol." The second construction is clearer in its meaning.

I don’t really get why one is in the wrong order? Maybe we’re parsing them differently somehow. The meaning reads identically to me although the parse tree is different.

Benzodiazepines have a similar effect to [the effect of] alcohol

Vs.

Benzodiazepines have an effect [which is] similar to [the effect of] alcohol.