A friend was taking losartan, but that didn't work. switched to valsartan. it worked. both in arb, why would the latter work instead?

Here are some ...: Check if she/he made some change in lifestyle such as decrease coffee drink or better sleep, etc. &/or added other hight-BP-related drugs; these may make BP control different. So, look into other factors so to sort out what might be. But occasionally, one may claim one is better than another for the drugs in the same class, like what happens in using Viagra, Levitra, (vardenafil) or Cialis in PDE-5is.