Blocked and clogged bathroom sink drains, and shower drains are typically caused by hair. Often times the P.O. plug and p-trap will contain a build-up of hair that will need to be pulled out. In shower drains, this is typically out of reach, and a drain auger will be required to remove the hair.
If you’ve got a blocked bathroom sink, nothing is more effective and safe than a physical drain cleaning.
If you hire a drain cleaning company, they’ll use special tools designed to physically clear the blockage and remove accumulated gunk from inside the pipe. This is superior to using chemical drain cleaners as chemical cleaners aren’t nearly as effective, they don’t clean out the interior pipe, and they’re damaging to your plumbing system.