Dozens of common medications can cause constipation, ranging from prescription painkillers to over-the-counter allergy pills and antacids. If you’ve recently started a new medication and noticed things slowing down, there’s a good chance the drug itself is the culprit. Here are the major categories to know about, how they affect your gut, and what you can do about it.
Opioid Pain Medications
Opioids are the single most common drug class linked to constipation, and it’s not a rare side effect. Between 40% and 95% of people on long-term opioid therapy develop constipation, and the risk increases the longer you take them. This includes prescription painkillers like oxycodone, hydrocodone, morphine, codeine, and fentanyl.
The reason opioids hit your gut so hard is that your intestines have the same receptors your brain uses to process pain relief. When opioids activate those receptors in your digestive tract, three things happen at once: your intestines slow their rhythmic contractions, your gut absorbs more water from stool (making it harder and drier), and your digestive secretions drop. Unlike many side effects that fade as your body adjusts, opioid-induced constipation typically does not get better on its own over time.
Anticholinergic Drugs
This is a broad category that catches many people off guard because it includes medications you might not associate with your gut. Anticholinergic drugs work by blocking a chemical messenger called acetylcholine, which your nervous system uses to trigger muscle contractions, including in your intestines. When that signal gets blocked, your bowel slows down.
The list of anticholinergic medications is long. Some of the most common ones include:
- Older antihistamines: diphenhydramine (Benadryl), chlorpheniramine, hydroxyzine, promethazine, and clemastine
- Overactive bladder medications: oxybutynin, tolterodine, solifenacin, darifenacin, and trospium
- Muscle relaxants: cyclobenzaprine and orphenadrine
Because older antihistamines are sold over the counter and widely used as sleep aids, many people take them nightly without realizing they’re contributing to constipation. Newer antihistamines like cetirizine (Zyrtec) and loratadine (Claritin) have far less anticholinergic activity and are much less likely to cause this problem.
Antidepressants
Tricyclic antidepressants like amitriptyline, imipramine, and nortriptyline have strong anticholinergic properties, which makes constipation one of their most predictable side effects. These older antidepressants are still prescribed for depression, chronic pain, migraines, and sleep problems.
Newer antidepressants can also slow your bowels, though generally less so. Among the SSRIs, paroxetine (Paxil) has the highest constipation rate, likely because it binds more strongly to the same receptors that tricyclics target. Overall, about 11% to 12.5% of people on SSRIs report constipation, with roughly 5% finding it bothersome enough to mention to their doctor. That’s a meaningful difference from the much higher rates seen with tricyclics.
Blood Pressure Medications
Not all blood pressure drugs cause constipation, but one class stands out: calcium channel blockers, particularly verapamil. Constipation affects more than 30% of people taking verapamil, with some studies reporting rates as high as 42%. Verapamil relaxes blood vessels by blocking calcium from entering smooth muscle cells, and the same mechanism slows the smooth muscle contractions in your intestines.
Other calcium channel blockers like diltiazem and nifedipine can also cause gastrointestinal issues, but verapamil ranks worst for constipation by a wide margin. If you’re on verapamil and struggling with regularity, this is worth discussing with your prescriber, since other blood pressure options may not carry the same risk.
Diuretics (Water Pills)
Diuretics treat high blood pressure and fluid retention by increasing how much water your kidneys send to your bladder. The trade-off is that less water is available for the rest of your body, including your colon. When your body is even mildly dehydrated, your intestines compensate by pulling more water from stool before it exits. The result is harder, drier stool that’s more difficult to pass.
Both loop diuretics (like furosemide) and thiazide diuretics can contribute to this effect. They also cause electrolyte shifts, including drops in potassium and magnesium, both of which your intestinal muscles need to contract properly. This combination of dehydration and electrolyte imbalance makes constipation more likely, especially in older adults who may already be drinking less fluid than they need.
Antacids and Stomach Medications
If you take antacids regularly, the ingredient list matters. Aluminum-based antacids are constipating, while magnesium-based antacids tend to cause diarrhea (magnesium hydroxide is literally the active ingredient in milk of magnesia). Most commercial antacid products combine both ingredients to balance these opposing effects, but if your particular brand leans heavier on aluminum, you may notice constipation.
Beyond antacids, other stomach-related medications can slow your gut too. Anti-nausea drugs like ondansetron (Zofran) are well-known for causing constipation, particularly when used repeatedly rather than as a one-time dose.
Iron and Calcium Supplements
Iron supplements are one of the most frequently overlooked causes of constipation. They’re commonly prescribed for anemia and taken by many people on their own, and constipation is one of the top reasons people stop taking them. Iron slows gut motility and can make stools harder, darker, and more difficult to pass. If iron is causing problems, taking it every other day rather than daily, or switching to a different form of iron, can sometimes help.
Calcium supplements, especially calcium carbonate (the type found in Tums and many store-brand supplements), can also contribute. Calcium citrate tends to be somewhat easier on the digestive system, though the difference varies from person to person.
What You Can Do About It
The most important step is identifying which medication is likely responsible. If you started a new drug and constipation followed within days or weeks, the connection is usually straightforward. When you’re on multiple medications from this list, the effects can stack, making the problem worse than any single drug would cause alone.
For most types of drug-induced constipation, over-the-counter laxatives are the standard first approach. Osmotic laxatives (like polyethylene glycol, sold as MiraLAX) work by pulling water into your intestines and are generally safe for regular use. Fiber supplements can also help, though they work best when you’re drinking plenty of water alongside them. Stimulant laxatives like bisacodyl or senna are effective for short-term relief but aren’t ideal as a daily long-term solution for most people.
Increasing your water intake matters more when the constipating drug is a diuretic or anything that dehydrates you, but it’s a reasonable baseline step regardless. Physical activity, even regular walking, helps stimulate bowel contractions.
For opioid-induced constipation specifically, the American Gastroenterological Association recommends starting with traditional laxatives. If those don’t work, there are prescription medications designed specifically to block opioid effects in the gut without reducing pain relief. These are worth asking about if standard laxatives aren’t doing enough.
In some cases, your prescriber can switch you to an alternative medication that’s less likely to cause constipation. This is especially practical with blood pressure drugs, antihistamines, and antidepressants, where newer options in the same class often carry a lower risk. Never stop a prescribed medication on your own because of constipation, but do bring it up at your next visit. It’s one of the most common and most fixable medication side effects.
Warning Signs to Watch For
Most drug-induced constipation is uncomfortable but manageable. However, certain symptoms suggest something more serious: rectal bleeding or blood on toilet tissue, black or tarry stools, persistent stomach pain that doesn’t let up, unexplained weight loss, or unusual changes in the shape or color of your stools. Constipation lasting more than three weeks or severe enough to interfere with daily life also warrants medical attention.