Are Drain Cleaning Products Like Liquid Plumr and Drano Safe?

As professional plumbers, one question that we hear frequently is, "are drain cleaning products such as Liquid Plumr and Drano safe to use on my pipes?" Well, the answer is not a simple yes or no. Let's take a look at the answer and some of the reasoning behind the answers so that you, as the consumer, can make an informed decision and decide for yourself if these types of products are appropriate for your particular situation.
Actually, these types of products, when used according to the manufacturer's recommendations, are generally safe for use in your home plumbing system. Just be sure that you do not plug the drain while you are using these types of products. As these products work their magic, a gas is emitted. This emission can put pressure on certain types of pipes if the drain is plugged while you are using drain cleaners. Specifically, schedule 40 PVC (Polyvinyl Chloride) and CPVC (Chlorinated Polyvinyl Chloride) pipes are thin and susceptible to damage. 
Some people worry that over a long period of time these types of products will "eat" away at your pipes. Honestly, it would take a massive number of uses to cause any damage to healthy pipes if used according to the manufacturers' recommendations. The risk is minimal and especially if you are using these products infrequently.
Sometimes the blockage may be caused by something as simple as hair. In this case, a drain cleaning agent may work to help dissolve the item that is causing the blockage and all will be well. But frequently these products will only dissolve the hair, or other foreign matter, into a clump. If a person wishes to try these products, go ahead. But if there is a recurring problem, call a licensed plumbing contractor in your area.
As plumbers who see these issues almost every day, we know that often the clog is a symptom and not the actual problem. And therefore, it is usually wise to have a licensed plumber take a look at the problem. Your stoppage could be caused by roots or another object. If this is the case, drain cleaning products are a waste of time and money. And depending on how far down your line the blockage is, you may likely need specialized plumbing equipment to help clear the stoppage and restore the normal flow of water down your drain.
The question isn't whether or not the products are safe, the question is "are the effective?" In many cases, the answer is no.