Does drain cleaner harm your plumbing? I have had mixed opinions. A plunger and a 10 foot snake does not always do the job. Thanks

I'm not a plumber but I've lived in houses for 20+ years using drain cleaner from time to time without ever damaging my plumbing.

Maybe it takes longer than 20+ years to do enough damage that you'd notice it?

Never had a problem with Drano products.
Read the label & don't use it in places that it says to avoid.

I have used it on and off for many, many years with no damage to my knowledge.

We have a cast iron main sewer line so I think I probably shouldn't use the caustic stuff. Most people now have PVC which lasts virtually forever and I don't think drain cleaner is going to hurt that stuff at all.

A clogged drain is worse.

Keep in mind you asked a general question and without parameters all we can do is give you general aka vague answers.

FWIW, there are "natural enzyme" cleaners that eat away the crud and are supposed to be "gentle".

Last time to clear a tub drain was about four hundred dollars. When the plumber arrives and clears the drain with a 30 foot snake he talks about how any of the drain cleaners will damage your pipes. That is as clear as I can be. The enzyme cleaners have been ineffective for me.

Wear safety glasses!!

Most drain cleaners ARE CORROSIVE. If your pipes are PVC it will not harm them. The drain cleaner usually comes in a plastic bottle.
Galvanized pipe is a different story. Where the threads are cut is the thinnest part of the pipe.
If the pipe does not have much fall or the pipe is plugged the chemicals will lay there and corrode the pipes.

If you have a SLOW drain it will be OK to use drain chemicals as per the directions on the bottle. Do NOT mix chemicals and more is NOT better.
On bath drains the clog is mostly caused by hair. The chemicals will not do much with hair but does disolve the soap scum, conditioners,tooth paste etc.

I have been snaking drains for 30+ years.

Drain cleaners are usually acids
PVC is good
Galvanized steel is bad
Black pipe is bad

Ok with PVC, however, it's not good on metal pipes and such.

On metal pipes the caustics will find the weak points, PVC is tough enough to take anything.

Acid can generate enough heat to melt plastic. Acid will eat right through older copper drains as the bottom of the pipe thins from erosion. Acid will take the spring out of a power drain cable and cause it to snap. Acid will eat the glaze off a porcelain tub.
Sunburn
Retired Plumber