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My opinion is simply that mechanical cleaning is always preferred, but when used cautiously and judiciously, chemical drain cleaners can be an effective substitute in SOME circumstances.
Note the keyword "some". Ask any plumber, and he (or she) will be able to cite numerous failures of drain cleaners. The reason has to do with the type of and extent of the blockage. Once a blockage becomes nested into your drain... especially if it is a clog related to hair... it is almost impossible for a chemical cleaner to free it. This is because the decomposed material causing the clog can actually protect itself from the action of the chemicals by its sheer size! In fact, a chemical cleaner can actually make a clog worse by turning the clog into a more solid, congealed mass. This can more effectively block the drain, hardly your goal!
Another circumstance where drain cleaners can cause more problems than they cure is when your pipes have become restricted due to long-term accumulations of goo. As in the above case, a drain cleaner can cause a partial loosening of the deposits which may cause a total blockage further down the line, especially if the flow through the pipes is slow!
The most dangerous aspect of drain cleaners is that they are powerful chemicals that can cause burns and even permanent eye damage! There are also possible chemical interactions if two different types are used, the worst being the release of suffocating and deadly chlorine gas! Using a caustic cleaner prior to mechanical cleaning can put you, your plumber or handyman at severe risk. THAT'S WHY it is UNWISE to use a drain cleaner in a totally stopped drain, EVEN THOUGH the pervasive ads for these products recommend it.
So my simple recommendations are as follows.
1) I do agree that monthly use of a drain cleaner is a good way to keep clogs from forming. Don't use a "professional" acid-based product for this purpose... they are meant for severe clogs only and are too dangerous for casual usage.
2) Once you notice the flow through the drain beginning to slow down, clean it right away. If you don't, the slowed flow of water through the drain will encourage a progressive build up of goo in drain pipes further down the line, since you have effectively decreased the movement of water through the whole system! This progressive buildup will eventually lead to an expensive and messy professional cleaning job!
If you are a handy do-it-yourselfer, you should be able to clear the blockage yourself. Most clogs form right in the pipes and trap directly beneath the sink. It is usually fairly easy to clear by one of two methods. First, remove the stopper so you can have a look down the drain. If the stopper does not come out, you will have to remove the stopper rod which is held in place by a large nut screwed onto the back of the drain below the sink and above the trap. (The rod, pipe and the activating mechanism are known as the "pop-up" assembly.) This rod extends horizontally into the drain and then through a hole in the bottom of the stopper, acting to move the stopper up and down. Be careful not to lose any plastic or rubber washers that might fall out when you remove this nut... they may be irreplaceable! Also, note the orientation of any "shaped" washers... if you install them backwards you may get water leakage around the rod when you reassemble the mechanism. Anyway, once the rod is pulled out, the stopper will be free.
Many times, there will be little else to do since the blockage may come right out with the stopper! Then again, if only a little stuff comes out or if the stopper just looks blackened and slimy, look into the drain with a flashlight. You may see more stuff in the drain a few inches down. If so, use a long screwdriver and carefully loosen the blockage. You can then flush out the goo by running a few sinkfuls of water through the drain. BE SURE TO COVER THE STOPPER ROD OPENING with your finger, sponge or rag... if you don't water will pour out of the hole and soak your cabinet! If the drain now runs clear, reassemble the stopper assembly and further flush the pipes by running a few sinkfulls of hot water down the drain.
If the blockage still exists, the second and more difficult cure is to disassemble the trap assembly under the sink. This will give you direct access to any blockage within the trap itself as well as access to the larger main pipes via a "snake". You might get lucky and find the blockage right within the trap or right where the sink drain pipes enter the wall. Worst case, you might need to use a plumbing snake... a flexible spring-like rod that is inserted into the pipe to break through and loosen blockages. A WARNING... if your pipes are old and corroded (sounds like an ad for Geritol!) they may break upon disassembly.
You might wonder why I didn't recommend using a plumbing snake in the trap. You could try it at your own risk. Sometimes it works but the force can break thin metal traps and some thin-walled plastic pipes. Sometimes the snake will not make the tight turn within the drain trap. So, as a rule, I prefer to disassemble the trap assembly and, if necessary, replace it and the associated connections with modern plastic pipes. They will not corrode and are easy to disassemble if, heaven forbid, the blockage recurs. This does not include any part of the popup assembly... just from the first "large" pipe nut under the popup assembly to the wall outlet.
Of course, if all these methods fail you may need the services of a professional to remove extensive blockages, especially if they extend beyond the confines of your home into underground drain pipes.