My mom got lasix pills, the one 40mg pill doesn't make her pee, but the two 20mg pills make her pee that are in another bottle. is it possible, the 40mg pill is bad or a placebo?

If the 40 mg pill was ordered and filled legally, by an MD and filled by a standard Pharmacy-it is not a placebo. She should not have two prescriptions of the same drug. Check with the prescriber about this.

I assume your mom had been prescribed Lasix 20 mg in the past but it was not strong enough so doctor increased the dose to 40 mg so she has both 20 and 40 mg tablets. A 40 mg tablet should be as effective as two tablets of 20 mg unless the 40 mg tablet is made by a different pharmaceutical company with poor quality and that may happen with a generic drug, but it is not a placebo.