The standard Miralax dose for adults is 17 grams once per day, which is one capful measured to the white line on the bottle’s measuring cap. That single daily dose is the recommended limit for self-treatment of occasional constipation. Higher doses are sometimes used under medical supervision, but taking more than 17 grams on your own is not advised.
The Standard Adult Dose
Miralax’s label is straightforward: one capful (17 grams) of powder mixed into 4 to 8 ounces of liquid, taken once a day. The cap doubles as a measuring tool, with a white section that marks the correct fill line. You can dissolve the powder in water, juice, coffee, or tea. It’s tasteless and odorless, so the drink you choose is mostly a matter of preference.
You should not use Miralax for more than seven consecutive days without guidance from a healthcare provider. It typically takes one to three days to produce a bowel movement, so don’t assume the first dose isn’t working if nothing happens within a few hours. Increasing the dose on your own to speed things up raises the risk of side effects without meaningfully helping.
How Miralax Works
Miralax is an osmotic laxative. The active ingredient, polyethylene glycol 3350, forms bonds with water molecules inside your intestines, preventing your colon from reabsorbing that water the way it normally does. The extra water stays in the stool, softening it and increasing its bulk. That triggers more natural contractions in the bowel, which is why it produces a gentler, more gradual effect compared to stimulant laxatives that directly irritate the intestinal wall.
Because the molecule itself is barely absorbed into your bloodstream, it stays almost entirely within your digestive tract. This is part of why it’s considered relatively safe at the recommended dose, but it also means that taking too much pulls excessive water into the colon, which can lead to dehydration and electrolyte problems.
When Doctors Prescribe Higher Doses
There are situations where a physician will instruct you to take significantly more than 17 grams in a single day. The most common is colonoscopy preparation. A typical bowel prep protocol involves mixing 238 grams of Miralax (about 14 capfuls) into 64 ounces of a clear sports drink like Gatorade, then drinking the entire mixture over the course of an evening and the following morning. That’s roughly 14 times the standard daily dose, consumed within a short window.
This is safe only because it’s a one-time event done under specific medical instructions, with careful attention to fluid intake. Even then, Miralax used this way does not contain built-in electrolyte replacement the way prescription bowel prep solutions do. The sports drink partially compensates, but it provides far less sodium than dedicated prep formulas, which is why some gastroenterologists prefer prescription alternatives for patients with kidney disease, heart failure, or other conditions where fluid and electrolyte shifts are risky.
For chronic constipation, especially in children, doctors sometimes adjust the daily dose upward from the baseline. Pediatric dosing at Stanford Medicine, for example, ranges from 0.4 to 1 gram per kilogram of body weight per day for maintenance. For a child dealing with a fecal impaction, the dose can go up to 1.5 grams per kilogram daily for three to six days before stepping back down. Johns Hopkins notes that providers may increase maintenance doses incrementally, adding a fraction of a capful every seven days up to a provider-determined maximum. These are individualized numbers that depend on the child’s weight, response, and the specific clinical situation.
Risks of Taking Too Much
The primary concern with exceeding the standard dose is dehydration. When too much water gets pulled into your colon, the rest of your body loses fluid. Symptoms of dehydration from laxative overuse include dizziness, dry mouth, reduced urination, and feeling unusually tired. If you’re already not drinking enough fluids, even the standard dose can leave you mildly dehydrated.
Electrolyte imbalances are the more serious risk. Unlike prescription bowel prep solutions that contain sodium, potassium, and other salts to offset intestinal losses, Miralax on its own does nothing to replace the electrolytes that leave your body with all that extra water. Taking large amounts without adequate electrolyte intake can lower your blood sodium levels, a condition that causes confusion, headaches, nausea, and in severe cases, seizures. This risk is particularly relevant for older adults, people taking diuretics, and anyone with kidney problems.
Bloating, gas, cramping, and diarrhea are the more common side effects even at normal doses. These tend to resolve once you stop taking it or your body adjusts, but they get worse at higher doses and can become genuinely uncomfortable.
What to Do if One Capful Isn’t Enough
If you’ve been taking one capful daily for two or three days without a bowel movement, resist the urge to double up. The one-to-three-day onset window is normal. Give it the full three days before concluding it isn’t working.
If a full week passes with little improvement, that’s the point to talk to a provider rather than increase the dose yourself. Persistent constipation that doesn’t respond to an osmotic laxative at the standard dose may have an underlying cause that more Miralax won’t fix, whether that’s a medication side effect, a thyroid issue, or a pelvic floor problem. A provider can safely adjust your dose, switch to a different approach, or investigate further.
In the meantime, increasing your water intake alongside Miralax makes a real difference. The drug works by holding water in the stool, so the more hydrated you are, the more effectively it can do its job. Drinking an extra two to three glasses of water throughout the day while using it is a simple way to improve results without changing the dose.