Metformin side effects are rare and it is usually well tolerated.  The most serious - and rare - side effect is lactic acidosis.  Here's six things to discuss with your doctor to determine your risk.

Before you get too worried about lactic acidosis from metformin you and your doctor should try to determine your risk.  First - what is lactic acidosis?  Basically, it's a condition in your body where your blood pH drops, creating an acidic environment.  There are other things that occur that are well beyond this conversation. But, there are signs and symptoms your should look out for if you think you might be getting lactic acidosis.

For instance, you might feel tired and weak, muscle pain, difficulty breathing, and stomach problems.  Granted, these are pretty broad symptoms and can make it hard to detect lactic acidosis.  That's why it's best for you and your doctor to cover the bases described below before you start metformin.

But, before we get to those, let me reassure you that your actual risk of getting lactic acidosis from metformin is extremely rare. In fact, it happens only to 1 out of every 33,000 patients.  However, when it does occur, it's fatal about 50% of the time.

So, in short.  Lactic acidosis is so rare I normally tell patients not to worry too much about it.  But, there are some patients who should be concerned.  Here's how to tell if you are one of them.

1) How is your kidney function?

What goes into your body must come out.  And with metformin it goes out via your kidneys. So, if your kidney function is poor (your doctor has to determine this with lab tests) you run the risk of not being able to eliminate the metformin from your body.  When that happens it continues to build up in your system.  And that's when you should start worrying about lactic acidosis.

2) Do you drink alcohol?

Drinking alcohol can increase the effect of metformin on lactate metabolism.  So, will one drink put you over the top? Obviously, that's something you and your doctor should talk about.  But, I think moderation is key.  Alcohol can have some therapeutic benefits in moderation. Unless you're a binge drinker or drink all the time this isn't too much of a concern.

3) How is your liver function?

As you read above, metformin is eliminated from your body through your kidneys, so why worry about liver function?  Because that's how lactate is eliminated from your body.  So, if you are producing lactate and don't want it to proceed to lactic acidosis you have to have a way to get rid of it.  That's why your liver needs to be in tip-top shape.

4) Are you going to have any contrast procedures done in the near future?

Sometimes your doctor may order a contrast procedure (like a CT scan with contrast).  Normally this involves the use of iodine.  These types of procedures can lead to changes in your kidney function. If you are going to do something like this make sure your doctor knows that you are on metformin.

5) What other medications are you taking?

The biggest risk of a drug interaction with metformin occur when you take Lasix® (furosemide), Lanoxin® (digoxin) and Procardia® (nifedipine). The end result of these drug interactions is that your metformin levels will be increased in your body, possibly leading to lactic acidosis.

6) How old are you?

Normally, as you age your kidney function begins to slow down. That's why if you are over 80 you must get your kidney function tested BEFORE starting metformin.

Also, even if you have a risk factor, you’re doctor may decide to keep you on the metformin anyways.  Why?  It is the only drug available that improves your glucose tolerance.  Also, since becoming generic it has dropped a lot in price. In short, you may have a tough time replacing such a worthy drug.