The standard Miralax dose for adults is 17 grams once a day, which is one capful or one single-dose packet. That single daily dose is the maximum recommended on the over-the-counter label, and the product is not intended to be taken more than once in a 24-hour period for routine constipation.
What One Dose Looks Like
The Miralax bottle cap doubles as a measuring cup. When you fill it to the white line inside the cap, you have exactly 17 grams of powder. You stir that into 4 to 8 ounces of any beverage, hot or cold, and drink it. Pre-measured single-dose packets also contain 17 grams. Taking a second capful in the same day is not part of the labeled instructions and increases the risk of side effects like diarrhea, cramping, and dehydration.
How Miralax Works
Miralax is an osmotic laxative, meaning it pulls water into your intestines rather than stimulating the gut muscles directly. The extra water softens the stool and adds bulk, which creates gentle pressure that helps your intestinal muscles move things along. Because it works by gradually drawing in fluid, it’s slower-acting than stimulant laxatives. It can take up to 24 hours to produce a bowel movement, and some people don’t see results for two or three days. That lag is normal and not a reason to double up on your dose.
Why Taking More Isn’t Better
If one capful hasn’t worked yet, adding a second dose the same day won’t speed things up. It will, however, pull significantly more water into your colon. The most common result is watery diarrhea and abdominal cramping. More concerning is the risk of dehydration and electrolyte imbalance, especially in children and older adults. Electrolytes like sodium and potassium regulate your heart rhythm, muscle function, and nerve signaling, so shifts in those levels can cause weakness, confusion, or irregular heartbeat.
People who misuse laxatives in large amounts, particularly those with eating disorders, face the most serious complications. Chronic overuse can lead to an inability to have bowel movements without laxatives, persistent electrolyte problems, and damage to normal intestinal function.
Higher Doses in Medical Settings
You may have heard of people drinking enormous amounts of Miralax before a colonoscopy. That’s a completely different situation. Bowel prep protocols use doses many times higher than 17 grams, but they’re prescribed and supervised by a doctor, taken over a short window with specific fluid guidelines, and designed to completely empty the colon for a procedure. These doses are not safe to replicate on your own for everyday constipation.
Pediatric hospitals also use weight-based dosing for children who need bowel cleanouts, with amounts that can range from 40 grams for a 22-pound child up to 180 grams for a child around 100 pounds. These are medically supervised protocols with strict maximum limits, not something to attempt at home without guidance.
How Long You Can Take It
The OTC label says to use Miralax for no more than 7 days. If constipation hasn’t resolved by then, that’s a signal to talk to a healthcare provider rather than continuing on your own. That said, doctors frequently prescribe Miralax for longer periods. The FDA reviewed safety data from clinical trials lasting 6 months and a full year before approving it, and many people with chronic constipation take it daily under medical supervision for extended stretches. The key difference is that longer use should be guided by a provider who can monitor for any issues.
Who Should Be Cautious
Miralax is not appropriate for everyone, even at the standard 17-gram dose. You should avoid it entirely if you have a bowel obstruction or intestinal blockage. People with kidney disease need to be especially careful because the kidneys are responsible for maintaining the fluid and electrolyte balance that Miralax can disrupt. Other conditions that warrant a conversation with your doctor before using Miralax include ulcerative colitis, irritable bowel syndrome, and any sudden change in bowel habits that has lasted two weeks or longer. Severe stomach pain with nausea or vomiting is another red flag, as these symptoms can indicate something more serious than simple constipation.
What to Do If One Dose Isn’t Working
Give it time. A full 24 hours is typical, and for some people the first bowel movement doesn’t come until the second or third day of daily use. Make sure you’re drinking enough fluids throughout the day, since Miralax works by drawing water into the colon and needs adequate hydration to do its job. If you’ve taken 17 grams daily for a full week without improvement, that’s the point to seek medical advice rather than increasing the dose yourself. Persistent constipation that doesn’t respond to an osmotic laxative can have underlying causes that need evaluation.