Is smelly urine ever a symptom of cancer?

If you’ve ever eaten asparagus, you know it can make your urine smell very unpleasant.  

But does anything else cause urine to stink? And, can smelly urine ever be a symptom of cancer?  

Read on to find out the answers to these and other questions. 

What causes urine to smell like fish or ammonia?

Ammonia is one of the waste products our bodies generate. It is removed primarily through the kidneys. All urine contains at least a small amount of this chemical compound. But when it’s diluted, the smell is not as noticeable. 

When we get dehydrated, the components that give urine its distinctive odor become concentrated. The ammonia smell can become quite strong. In large enough concentrations, it can even start to smell fishy, because ammonia is the same compound that makes rotting fish smell the way it does. 

Think of it this way: if you go for a long jog on a hot day and notice your urine is darker and has a powerful ammonia smell, that’s your body giving you a clear signal: “I need water!” 

What causes urine to smell sweet or fruity?

Diabetes mellitus is often the culprit when urine smells sweet or fruity. That’s because when your blood sugar levels get too high, your kidneys start dumping it into the urine and prompting you to urinate more often. 

In fact, diabetes was often diagnosed in ancient times by actually tasting a patient’s urine. The word “diabetes” can be roughly translated as “runs straight through,” while the word “mellitus” means “sweet.”  

A persistently sweet or fruity urine smell can sometimes be the first physical sign a person with undiagnosed diabetes notices. So, if you experience that, or increased thirst and frequent urination, contact your doctor.  

What else can cause urine to smell nasty?

Urinary tract infections (UTI)

Assuming it’s not due to diet or dehydration, the most common cause of foul-smelling urine is a urinary tract infection, or UTI. One of the reasons stale urine smells so bad is that bacteria start breaking down the ammonia it contains. With a bladder infection, that process takes place inside your body, which can lead to smelly urine.  

Many people, especially women, have experienced a UTI. The accompanying strong, often foul, odor can be so distinctive that it immediately flags the issue. It’s important to seek treatment quickly before the infection progresses. 

Sexually transmitted diseases

Sexually transmitted diseases may also give urine a foul odor. That’s because they attack the urethra and prompt it to generate secretions that get mixed in with the urine. 

Medications and supplements

Some medications can change the smell of your urine, especially one class of diabetes medications called sulfonylurea drugs. This doesn’t happen to everyone, though, just some people. And, theoretically, anything you ingest can change the smell of your urine. So, if you recently changed medications or started taking a new dietary supplement, that might be the reason.  

Metabolic disorders

Rare metabolic disorders, such as phenylketonuria, in which the body doesn’t process ketones properly, can also change the smell of your urine. But that’s usually only in really severe cases.