First and foremost, when someone has a drain clog, it can be very frustrating. The most common clog is the simple toilet back up. This happens when your toddler decides that five rolls of toilet paper should go down the drain, and then proceeds to flush until you have a mess on the floor. Other clogs usually accumulate from hair mixed with soap, grease, or any other foreign matter sent down a drain. Water backs up at these points, and then your tub fills up with unspeakable horrors.
Naturally, most homeowners want the cesspool of hate in their tub to leave as quickly as possible, so instead of spending the hundreds of dollars on a plumber, they will run out to Walmart and buy a bottle of caustic cleaner, like Drano, Liquid Plumr, or another brand.
Why are these things terrible? Drano has a PH of around 11, is a strong alkaline, and causes some pretty severe chemical reactions. where It's not that big of a deal when the alkaline is going down your drains, it is a big deal if you have to get a plumber involved.
Liquid drain clearing solutions are typically a composite of lye, bleach, and other chemicals. They can contain aluminum or other metallic shavings. Lye is so caustic that it can cause extreme chemical burns. Lye also creates an exothermic reaction when mixed with water. This is one of the ways liquid drain cleaners clear clogs, is once water is combined with the solution, it creates thermal based pressure, which pushes the clog down the line. That thermal expansion can create havoc on glue joints, and possible increase separations in your sewer line. Heat is also bad for most sewer lines, as many drains consist of a thinner walled PVC called DWV pipe. DWV can break if the pipe is heated above 140 degrees Fahrenheit, and lye (the primary chemical in most liquid drain clearing solutions) produce temperatures in excess of 200 degrees Fahrenheit when mixed with water.
Liquid drain clearing solutions are notoriously unpredictable when mixed with other cleaning solutions. Remember when your mom told you never to mix ammonia and bleach? Well, there is a good reason to ensure you aren't using any other cleaning solutions when you run Drano down your toilet. Gasses that can be formed during the clog removal primarily include H2, which while flammable, most likely won't do much when it dissipates into the air, however, if a belly in your sewer line caused the blockage, and the Drano hits concentrated urine (a not implausible situation), you get Chlorine gas. While the chances of that gas finding its way into a house in such sufficient quantity as to cause harm is very low, it is better not to take the chance.
Last but not least, chemical drain cleaners are typically found in liquid form. They splash around when poured, and can get everywhere. Unlike bleach, which turns your nice towels white, chemical drain cleaners can leave lasting scars.
Bottom line, use a plunger, buy a cheap auger ($22.49 at Lowes at the time of this writing), or call a plumber, you won't regret it, and you will get back to your bath with little delay.