What Medications Are Bad for Septic Systems?

Your septic system is comprised of three main ingredients, all mixed into a stew of poo that we’d rather not talk about! In a balanced septic tank, these mixtures usually end up forming three different layers. Human waste solids and food waste drop to the bottom of the tank and settle there. Grease, hair and other lighter substances float on the top level. The liquid in between the two is called “effluent”.

Sound like fun?

While there are tons of things that can go wrong with a septic system, we are going to focus on one that you can control. Let’s talk about medications that can mess up your “stew” and cause some gross blockages in your system. At the end of this article, we’ll link to another great read: How to Mess Up a Septic Tank.

Why do Medications Harm Septic Systems?

Medications are harmful to your septic system because they can kill or alter the bacteria that are inside the tank. You might think this is a good thing, and in most cases it is… except in the case of a septic system.

Bacteria exist in a septic system for the sole purpose of breaking down the solid waste that is deposited into the system. Without these bacteria, the system can become clogged with unprocessed human waste which, in turn, can cause some major issues for you and your neighbors.

One of the issues is horrible odors drifting into your home and around your property. If you are experiencing this, consider a maintenance routine that adds the right right amount of aerobic bacteria and oxygen. Products like Septifix (link to product) can be a blessing in between your tank pumpings.

Another likely issue is gray water (unprocessed sewage) backing up into your sinks, showers and toilets. This dirty water can also infiltrate the leech field and seep into the groundwater and cause contamination.

Which Medications Are the Worst for Septic Systems?

Antibiotics, medicines for bacterial infections and chemotherapy drugs are especially harmful to the bacteria in your septic system.  If the concentration of these drugs is too high inside the septic tank, they can kill the bacteria responsible for breaking down raw sewage.

If the bacteria can’t break down the solid waste, it can build up and cause all of the problems listed above.  Here are some common antibiotic types that should NEVER be flushed into a septic tank:

  • Tetracyclines
  • Penicillins
  • Cephalosporins
  • Quinolones
  • Lincomycins
  • Macrolides
  • Sulfonamides
  • Glycopeptides
  • Aminoglycosides
  • Carbapenems

How do Medications Get into a Septic System?

The two main ways that medicines get into your septic system are through human waste and through intentional disposal.

Human Waste

This is not a usually a problem when you have a right sized septic tank and maintain it properly.  But if you have more people using a tank that what it’s rated to handle, and if some of those people are on high doses of daily medications, it could lead to an imbalanced septic tank. 

Intentional Disposal of Medications

Whether you’re dealing with a medicine cabinet full of expired prescriptions, or cleaning out an estate home.  Many people assume it’s safest to flush medications down the toilet when they are no longer needed.

This method of intentional, mass disposal is likely to cause a septic system to go out of balance rapidly because the doses would be extremely high and concentrated.  And in the cases of estates or vacant homes, there would be no water entering the system to help dilute the medications.


Your septic system needs a delicate balance of waste and “good” bacteria to break down that waste. The addition of chemicals such as unused medications can upset the ecosystem in the tank and cause issues for you and your neighbors.

Want to learn more ways that you can actually be hurting your septic system? Move on to the next article to find out all the DIYs you should STOP DIY’ing when it comes to your septic tank: How to Mess Up a Septic Tank.

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