Common Opioid Side Effects: Constipation, Drowsiness, and Nausea

When opioids are prescribed for severe pain-after surgery, for cancer, or in cases where other treatments fail-they work. But they also bring along a set of unwelcome guests: constipation, drowsiness, and nausea. These aren’t rare or mild reactions. They’re predictable, common, and often under-addressed. For many people, these side effects don’t fade with time. They stick around, making daily life harder and sometimes pushing patients to stop taking their medication-or worse, to misuse it.

Constipation: The Most Common Side Effect You Can’t Ignore

Almost everyone who takes opioids long-term gets constipated. It’s not just inconvenient-it’s a medical issue that can lead to bowel obstruction, hemorrhoids, or even hospitalization. Why? Opioids bind to receptors in your gut, slowing down the natural muscle contractions that move food through your system. At the same time, they make your intestines absorb more water, turning stool hard and dry.

Unlike drowsiness or nausea, which often improve after a few days, constipation doesn’t get better on its own. If you’re on opioids for more than a week, you need to act before it becomes a problem. Waiting until you haven’t had a bowel movement in three days is too late.

Doctors should start you on a laxative regimen the same day you begin opioid therapy. The most effective approach combines a stimulant laxative like senna with an osmotic agent like polyethylene glycol (Miralax). These work together: one wakes up your bowels, the other pulls water into them. Over-the-counter options often work fine, but if they don’t, prescription drugs like methylnaltrexone (Relistor) or naloxegol (Movantik) target only the gut-leaving pain relief intact.

Don’t rely on prune juice or fiber supplements alone. They’re not enough. And never stop your opioid because of constipation. There are better solutions.

Drowsiness: When Your Brain Feels Like It’s Underwater

Feeling sleepy after taking an opioid? You’re not alone. Between 20% and 60% of people experience drowsiness when they first start using these medications. It’s not laziness. It’s the drug acting on your brainstem and cortex, slowing down neural activity. This isn’t just about feeling tired-it’s mental fog, slowed reaction times, and trouble concentrating. For some, it’s like walking through wet cement.

The good news? For most people, this fades within a week or two as the body adjusts. But for 10% to 15% of long-term users, drowsiness never fully goes away. And that’s dangerous. Driving, operating machinery, or even walking down stairs becomes risky. Older adults and those with existing sleep apnea or cognitive issues are especially vulnerable.

Here’s what helps: Start with the lowest possible dose. Take your opioid at night if you can, so the drowsiness hits when you’re already planning to rest. Avoid alcohol, benzodiazepines (like Valium or Xanax), or sleep aids-they multiply the effect and can cause fatal breathing problems. If drowsiness lingers and interferes with your life, talk to your doctor about switching opioids or adding a low-dose stimulant like methylphenidate. There’s limited evidence, but some patients report real improvement.

Don’t ignore this symptom. If you’re nodding off during the day, it’s not normal. It’s a signal your body can’t handle the current dose-or you’re on the wrong medication.

Nausea: Why Your Stomach Feels Like It’s in Revolt

Nausea hits about one in three people starting opioids. It’s not just queasiness-it’s vomiting, sweating, dizziness, and a constant feeling that your stomach is trying to escape. Opioids trigger this in two ways: they irritate the brain’s vomiting center (the chemoreceptor trigger zone), and they slow stomach emptying, leaving food sitting there too long.

For most, nausea eases after 3 to 7 days. But for 1 in 10 long-term users, it sticks around. And that’s enough to make someone quit their pain medication-even if they’re still in pain.

Treatment depends on what’s driving the nausea. Dopamine blockers like metoclopramide or prochlorperazine are often the first choice because they target the brain’s vomiting center. Ondansetron (Zofran), which blocks serotonin, works well for some people, especially if nausea is tied to delayed gastric emptying. Antihistamines like promethazine can help too, but they add more drowsiness, which isn’t helpful if you’re already feeling foggy.

Timing matters. Take antiemetics 30 minutes before your opioid dose. Avoid heavy meals. Stay upright after taking your pill. And if nausea returns after weeks of being gone? It could be a sign your body is developing tolerance-or that you’re starting to withdraw.

Never stop opioids cold turkey. Sudden discontinuation can trigger violent nausea, vomiting, diarrhea, and even suicidal thoughts. Always taper under medical supervision.

Why These Side Effects Matter More Than You Think

These three side effects-constipation, drowsiness, nausea-are more than just annoyances. They’re the main reasons people stop taking opioids, even when they still need them. And when people stop abruptly, they often turn to street drugs to manage their pain. The CDC reports that more than two-thirds of people who misuse prescription opioids do so because they’re trying to relieve physical pain.

That’s the irony: the very drugs meant to help with pain become the reason people seek dangerous alternatives. Poor side effect management fuels the opioid crisis-not just through addiction, but through desperation.

Doctors are learning this. Guidelines from the American Academy of Family Physicians and the Faculty of Pain Medicine now stress that side effects should be anticipated, not reacted to. Prophylactic laxatives. Scheduled anti-nausea meds. Dose titration over time. These aren’t extras-they’re part of the treatment plan.

And patients need to speak up. If you’re constipated, say so. If you’re too drowsy to work, say so. If you’re throwing up every morning, say so. There are solutions. You don’t have to suffer in silence.

What to Do If You’re Struggling

• For constipation: Start laxatives on day one. Use senna + polyethylene glycol. Drink water. Move your body. Ask about Relistor or Movantik if OTC doesn’t work.
• For drowsiness: Take opioids at night. Avoid alcohol and sedatives. Don’t drive until you know how you react. Talk to your doctor about lowering the dose or switching to a different opioid like buprenorphine, which causes less sedation.
• For nausea: Try metoclopramide first. Take it 30 minutes before your opioid. Avoid greasy food. If it persists, ask about ondansetron or a different opioid class.

Also, review all your other medications. Some antidepressants, antihistamines, and muscle relaxants make drowsiness worse. Your pharmacist can help spot interactions.

The Bigger Picture: Safer Pain Management

Opioids aren’t the only option. For chronic pain, physical therapy, nerve blocks, cognitive behavioral therapy, and non-opioid painkillers like gabapentin or acetaminophen can reduce opioid needs. Even small reductions in opioid dose can dramatically cut side effects.

And new drugs are coming. Researchers are developing biased opioid agonists that relieve pain without triggering the pathways that cause nausea or constipation. Peripherally-acting antagonists like naldemedine (Symproic) are already in use for constipation. The future is about smarter opioids-not just stronger ones.

But for now, the best tool is awareness. If you’re on opioids, expect these side effects. Plan for them. Treat them early. And never suffer alone.