When it comes to stool, lab technicians have arguably one of the worst jobs in the business. Even if it may not be one of the most pleasant tasks to help treat a patient, they provide an invaluable service because of how important stool samples really are to assess the health of a patient. If you have noticed that the aroma of your stool has changed to a metallic smell, it could indicate a number of different pathologies that you need to be aware of.
The foods that you eat can play a big role in the smell of stool. Greasy or fatty foods can make foul-smelling gas but what generally leads to a metallic smell could be from supplements or diet pills that you may be taking. Due to the preservatives and color dyes found in many of the vitamin pills that are commonly taken in modern society, an unfortunate side effect will be this metallic smelling stool. Vitamin B supplements and garlic supplements are two known compounds that can lead to this side effect.
Another thing that you need to be aware of when you are trying to discover what could be causing your metallic smell could be excess iron in your stool. One of the most worrisome reasons that this could happen would be from internal bleeding, likely from your intestines. If you find that your stool is darker in color than typical, this could make this diagnosis even more likely. If you notice this darkening in your stool, it is imperative for you to make your way to a doctor to check your stool for blood. The most worrisome conditions that could cause this would be from colorectal cancer. You will need to have a fecal occult blood test testing for trace amounts of blood in your stool and potentially also a colonoscopy looking for gross transformations.
Some medications can also lead to this metallic smell. Commonly prescribed antibiotics like metronidazole have been reported as one to cause this metallic smell. It is up to you to notice any apparent changes after taking medications. In most cases, it will be just an unfortunate side effect that will not add any further risk but bring up this concern with your doctor to make sure he is aware of these side effects.
The last possible cause of metallic smelling stools would again be based on your diet. Studies indicate that this is a common occurrence in people using a Paleo diet. This is a diet focused on having high levels of healthy fats while minimizing the carbohydrates. As your body adjusts to this new diet, it is only natural for you to start to produce more ketone bodies as a by-product of your metabolism. Ketone bodies can create this metallic smell as well when you are passing gas.
All in all, it is important for you to go to a doctor to rule out long-term conditions that can be life-threatening. Once this has been ruled out, metallic smelling stools will self-resolve on its own in most cases. 
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