The best medicine for constipation depends on how quickly you need relief and what’s causing the problem. For most people, a fiber supplement or an osmotic laxative like polyethylene glycol (sold as MiraLAX and generics) is the place to start. These are the options backed by the strongest evidence, and a joint guideline from the American College of Gastroenterology and American Gastroenterological Association gives polyethylene glycol its highest recommendation for chronic constipation in adults.
But there are several types of laxatives on store shelves, and they work in very different ways, on very different timelines. Here’s how to sort through them.
Fiber Supplements: The Gentlest Starting Point
Fiber supplements are bulk-forming laxatives. They absorb water into your stool, making it larger and softer, which triggers your colon to contract and push things along. Psyllium (sold as Metamucil and generics) is the most studied option, and the FDA allows adult doses of up to 30 grams per day, typically taken in divided doses of 2.5 to 7.5 grams at a time with a full glass of water.
The tradeoff is patience. Fiber supplements take about 24 hours to start working and reach their full effect in two to three days. They’re better suited for ongoing constipation management than acute relief. If you’re not getting enough fiber from food, most women should aim for 25 to 30 grams per day and most men for 30 to 38 grams, combining food sources and supplements as needed. Start slowly and increase gradually, because adding too much fiber at once can cause bloating and gas.
Osmotic Laxatives: The Strongest Evidence
Osmotic laxatives pull water into the colon to soften stool. Polyethylene glycol 3350 (MiraLAX) is the most widely recommended option in this category. It’s tasteless and odorless, mixes into any beverage, and carries a strong recommendation from gastroenterology guidelines. Timing varies, but relief typically comes within one to two days.
Lactulose is another osmotic option, available by prescription in many countries, with a similar one-to-two-day onset. Magnesium-based options work differently within this class. Magnesium oxide recently received a conditional recommendation for chronic constipation, and magnesium sulfate can produce a bowel movement in as little as one hour, making it one of the faster oral options. Be cautious with magnesium products if you have kidney problems, since your body may struggle to clear the excess.
Stimulant Laxatives: Fast but Not First
Stimulant laxatives activate the nerves controlling your colon muscles, forcing contractions that move stool through. Senna (sold as Senokot and generics) and bisacodyl (Dulcolax) both work within 6 to 12 hours when taken by mouth. Bisacodyl suppositories work much faster, typically within 15 to 60 minutes.
These are effective when fiber and osmotic laxatives haven’t done enough. The 2024 ACG/AGA guideline gives senna a conditional recommendation, making it the first time a major gastroenterology guideline has endorsed it as an evidence-based treatment. The old concern that stimulant laxatives cause colon cancer has not held up. A large prospective study of over 84,000 women found no association between laxative use and colorectal cancer, and the overall evidence now suggests stimulant laxatives are not an independent risk factor.
That said, stimulant laxatives can cause cramping and, with prolonged use, may lead to dehydration or electrolyte imbalances. They’re best used for short-term relief rather than as a daily long-term strategy, unless your doctor advises otherwise.
Stool Softeners: Common but Weak Evidence
Docusate sodium (Colace) is one of the most commonly purchased constipation remedies, but the evidence behind it is surprisingly thin. The American Gastroenterological Association does not include docusate as a recommended treatment. A 2021 review found it showed no benefit compared to placebo, and direct comparisons show that both psyllium and senna outperform it.
Stool softeners take one to three days to work and increase the water and fat your stool absorbs. They won’t hurt you, but if you’re choosing between a stool softener and a fiber supplement or osmotic laxative, the evidence favors the latter two.
Suppositories and Rectal Options
When you need relief quickly, rectal options work fastest. Glycerin suppositories typically produce a bowel movement within 5 to 30 minutes. Bisacodyl suppositories take 15 to 60 minutes. Sodium phosphate enemas can work in as little as 2 to 5 minutes.
These are useful for occasional situations where oral laxatives haven’t worked or you can’t wait hours for them to kick in. Glycerin suppositories are gentle enough for children and infants, though they shouldn’t be used daily because they can cause rectal irritation and your body may become dependent on them.
What to Use for Children
For kids, osmotic laxatives are the preferred first-line treatment. Polyethylene glycol is the top choice among pediatric gastroenterologists because of its tasteless, odorless profile, which makes it easier to mix into drinks without a battle. Stool softeners and magnesium hydroxide (Milk of Magnesia) are also options, though magnesium hydroxide should be used cautiously in children with kidney issues.
Stimulant laxatives like senna and bisacodyl are generally reserved for children with more severe constipation who haven’t responded to diet changes and gentler medications. Multiple laxatives should not be given to a child at the same time unless a pediatrician approves it.
How to Choose the Right One
Your decision comes down to three factors: how urgent the situation is, whether you’re dealing with a one-time episode or an ongoing pattern, and what you’ve already tried.
- Mild or ongoing constipation: Start with a fiber supplement like psyllium, taken daily with plenty of water. Give it two to three days to reach full effect.
- Moderate constipation that fiber hasn’t fixed: Add or switch to an osmotic laxative like polyethylene glycol. This is the option with the broadest guideline support.
- Need relief tonight: An oral stimulant laxative (senna or bisacodyl) taken before bed will typically work by morning.
- Need relief now: A glycerin or bisacodyl suppository will work within an hour, often much sooner.
- Chronic constipation that doesn’t respond to OTC options: Prescription medications like linaclotide, plecanatide, and prucalopride all carry strong recommendations from gastroenterology guidelines.
Warning Signs That Need Medical Attention
If constipation lasts longer than a week, it’s worth scheduling a doctor’s appointment. Go to the emergency room if you haven’t had a bowel movement for a prolonged period and you’re also experiencing severe abdominal pain or major bloating. Other warning signs that warrant prompt medical evaluation include vomiting, blood in your stool, and unexplained weight loss.