GoLYTELY works by pulling water into your intestines through osmosis, flushing out their contents until your colon is clean enough for a colonoscopy. The solution contains a large polymer molecule (polyethylene glycol 3350) that your body can’t absorb. Because this molecule stays in your gut, it drags water along with it, creating a powerful wave of liquid that pushes everything through.
The Osmotic Mechanism
The key ingredient in GoLYTELY is polyethylene glycol 3350, a long-chain molecule that’s too large to pass through the walls of your intestines. When you drink the solution, this molecule stays inside the intestinal tract and holds water around it. Your body can’t pull that water back, so instead of being absorbed like it normally would, the fluid accumulates and moves forward, sweeping solid waste out ahead of it.
This process is called osmotic laxation. Your intestines normally absorb most of the water you drink, but polyethylene glycol essentially overrides that system. The result is a large volume of liquid flowing through your entire digestive tract, producing frequent watery bowel movements that progressively clear the colon. Bowel movements typically start within 30 to 60 minutes after you begin drinking the solution.
Why It Contains Electrolytes
GoLYTELY isn’t just polyethylene glycol and water. Each gallon also contains sodium sulfate, sodium bicarbonate, sodium chloride, and potassium chloride. These electrolytes serve a specific purpose: they keep the solution balanced with your body’s own fluids so that drinking a gallon of liquid doesn’t dangerously shift your sodium, potassium, or other mineral levels.
Without these added electrolytes, pushing that much fluid through your system could cause dehydration or electrolyte imbalances. The formula is designed so the solution passes through you without pulling minerals out of your bloodstream or dumping excess minerals in. This is what makes GoLYTELY safer for full-volume bowel prep compared to some other laxative approaches, though side effects still occur.
What the Prep Involves
GoLYTELY comes as a powder that you mix with water to make a full gallon (about 4 liters) of solution. You drink it in 8-ounce glasses, typically at a steady pace over several hours. Many doctors now recommend a split-dose approach: you drink half the solution the evening before your colonoscopy and the other half early the next morning. This tends to be easier to tolerate and produces a cleaner colon than drinking it all at once.
The day before your procedure, you’ll need to follow a strict clear liquid diet. This means no solid food and no dairy. You can have tea, black coffee (morning only), apple juice, white grape juice, clear broths, sports drinks, gelatin, popsicles, and flavored waters. The critical rule is avoiding anything red, blue, or purple, because those dyes can coat the colon lining and look like blood or abnormal tissue during the colonoscopy.
Common Side Effects
Nausea, bloating, and abdominal fullness are the most common side effects, affecting up to 50% of people who take GoLYTELY. This makes sense when you consider you’re drinking a gallon of salty, slightly viscous liquid in a relatively short window. Cramping is also common as the fluid moves through your intestines.
A few strategies can help. Chilling the solution makes it easier to drink. Some people find that sucking on a lemon wedge or hard candy between glasses cuts the taste. Drinking at a steady pace rather than chugging large amounts at once reduces nausea. If you vomit, waiting 30 minutes and then restarting at a slower pace usually works. The bowel movements themselves are watery and frequent, so staying close to a bathroom once you start drinking is essential. Most people find the active purging phase lasts a few hours after each drinking session.
Why a Clean Colon Matters
The entire point of GoLYTELY is to give your doctor an unobstructed view of your colon’s lining. Even small amounts of remaining stool can hide polyps, which are the precursors to colorectal cancer. A poor prep doesn’t just make the procedure harder; it can mean your doctor misses something important, or you may need to repeat the colonoscopy sooner than you otherwise would have. Studies consistently show that the quality of the bowel prep is one of the strongest predictors of whether a colonoscopy catches what it’s supposed to catch.
By the end of the prep, your bowel movements should be clear or light yellow with no solid material. That’s the signal that the solution has done its job and your colon is ready for examination.