Well, there's a question everyone thinks has a simple answer, that really doesn't.
Believe it or not, there's no "rule" or "regulation" about what can be called a banana bag, or what something called a banana bag has to include. MOST reputable medical facilities don't call anything a banana bag with regularity (at least officially), unless they have a stated procedure about it. However, almost all places that infuse something they call a banana bag include at least three things: folic acid (vitamin B9), thiamine (vitamin B1), and magnesium, and most include a low-potency multivitamin.
In general, in a hospital setting
banana bags are given to chronic alcoholics, people who have had weight loss surgery, terminally ill patients, or those who have experienced a temporary physical issue that has led to loss of fluids and caused physical pain.
Thiamine is included in banana bags, among other reasons, to prevent a syndrome called Wernicke's Disease, which affects both long term alcohol abusers and those who have had weight loss surgery. It's also used to fight the affects of a deficiency caused by a diet that doesn't include enough grains and/or legumes. Both issues are prevalent in those two demographics, and can lead to Wernicke's (which can cause vision issues up to and including blindness). It's VERY effective for people who haven't eaten properly, who (like many of us around here) eat a mostly meat-centered diet, or who have skipped meals regularly.
Folic acid (the manufactured form of Folate, or B9) is used to fight anemia. It's also required by the body for RNA and DNA production and cellular division. It can't be made BY the body, so it HAS to be absorbed through food. Those who haven't gotten proper nutrition are subject to Folic acid deficiency, which can lead to abnormally enlarged red blood cells, and a host of issues. Chronic tiredness, heart palpitations (atrial fibrillation and atrial flutter), and an increased risk of stroke are all possibilities when someone is folic deficient.
Magnesium is a natural muscle relaxant that works wonders with muscle cramps, spasms, tremors, muscle pain, and heart palpitations.
Because most people who need a banana bag haven't, at least recently, been managing a well-balanced diet, a low-potency multivitamin that includes a few B vitamins is also usually included.
Because our "banana bag" goes above and beyond, we don't refer to it as a banana bag. We call it The Lagniappe. "Lagniappe" is a Cajun word that means "a little something extra".
The "extra" for us is multi-fold. We use Ringer's Lactate instead of Normal Saline. Ringer's includes potassium (great for muscle pain, restless leg syndrome, "charlie-horses", and other muscle cramp issues), and calcium, whereas Normal Saline is nothing more than water and salt. It's also isotonically more similar to blood than normal saline.
As importantly, instead of a low-potency dose of a few B vitamins, we use the most potent multivitamin we've been able to find, Infuvite by Baxter. It includes ALL of the following:
Vitamin A (for cellular growth, which may be an issue for those getting a banana bag, immune system strengthening, and vision improvement)
Vitamin B12 (for energy)
Vitamin C (for its anti-inflammatory effects)
Vitamin D (for immunity and calcium absorption)
Vitamin E (primarily for blood and brain health, though good for skin and vision)
Vitamin K1 (liver health, also a coagulant)
All of THAT is why we don't call The Lagniappe a "banana bag".
Last, but not least, everyone asks why the banana bag is called that, and what makes it yellow. Many people mistakenly assume that potassium in the bag turns the fluids yellow, because they know bananas are high in potassium and think that potassium must be what makes them both yellow. As you now know, most banana bags have no potassium at all. It's the multivitamin that makes the bag yellow. The name comes from the yellow color, not from any particular ingredient.
Now you know what a banana bag is. Anyone can call any IV bag full of fluids that turns yellow a "banana bag". Before you get one anywhere other than Rapid Recovery make sure you ask EXACTLY what's in your bag.