Common Opioid Side Effects: Constipation, Drowsiness, and Nausea

When opioids are prescribed for severe pain-after surgery, for cancer, or due to chronic injury-they work. But they also come with a set of side effects that don’t go away just because you get used to them. Constipation, drowsiness, and nausea are the most common, and they affect most people who take these drugs long-term. Ignoring them doesn’t make them disappear. In fact, leaving them unmanaged can lead to stopping your medication altogether, worsening pain, or even dangerous withdrawal.

Constipation: The One Side Effect That Never Goes Away

Almost everyone who takes opioids for more than a few weeks will get constipated. It’s not rare. It’s expected. Opioids slow down your gut by binding to receptors in your intestines, making stool move slower and water get sucked out. The result? Hard, painful bowel movements that don’t improve over time-unlike drowsiness or nausea, which often fade.

Doctors used to wait until patients complained before acting. That’s outdated. The American Academy of Family Physicians recommends starting a laxative regimen on day one of opioid therapy. Don’t wait for symptoms. Prophylaxis works. A mix of a stimulant laxative like senna and an osmotic agent like polyethylene glycol (Miralax) is the standard first-line approach. These help keep things moving without interfering with pain relief.

If that doesn’t cut it, there’s a class of drugs called PAMORAs-peripherally acting mu-opioid receptor antagonists. Medications like methylnaltrexone (Relistor) and naloxegol (Movantik) block opioid effects in the gut but don’t cross into the brain. That means your pain control stays intact while your bowels start working again. These aren’t over-the-counter fixes. They’re prescription tools for people who’ve tried everything else and still can’t go.

Drowsiness: When Your Brain Feels Like It’s Underwater

When you first start opioids, you might feel like you’re walking through molasses. Your thoughts get foggy. You yawn constantly. You struggle to stay awake during the day. That’s drowsiness-and it’s real. Between 20% and 60% of new opioid users experience it, according to clinical studies. For most, it fades within a week or two as the body adjusts. But for 10% to 15% of long-term users, the fog never lifts.

It’s not just inconvenience. It’s risk. Drowsiness increases the chance of falls, car crashes, and mistakes at work. It also makes cognitive tasks-reading, remembering names, following instructions-harder. This is especially dangerous for older adults or those with existing brain changes.

Management starts with timing. Take your dose at night if possible. Avoid driving or operating machinery until you know how you react. Cut out other sedating meds like benzodiazepines or alcohol-they multiply the effect dangerously. If drowsiness lingers, talk to your doctor about lowering the dose. Sometimes, switching to a different opioid helps. There’s limited evidence for stimulants like methylphenidate (Ritalin) to fight opioid-induced sleepiness, but they’re rarely used due to side effects and lack of strong data.

Don’t ignore persistent drowsiness. It’s not just fatigue. It’s a sign your body might be struggling with the dose-or that you’re at higher risk for respiratory depression, the leading cause of opioid overdose death.

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

Nausea hits about one in four people when they start opioids. It’s not just an upset stomach. It’s the drug directly stimulating the brain’s vomiting center, plus slowing digestion so food sits there too long. For most, it fades within a few days. But for 1 in 10 long-term users, it sticks around-and it’s enough to make them quit their medication.

There are several types of anti-nausea drugs, each targeting a different pathway. Dopamine blockers like metoclopramide and prochlorperazine are usually tried first. If they don’t help, serotonin blockers like ondansetron (Zofran) are next. Antihistamines like promethazine work for some, especially if dizziness is also part of the problem. It’s trial and error. What works for one person might do nothing for another.

Don’t take these drugs long-term without supervision. Some, like metoclopramide, can cause movement disorders if used for months. Also, don’t assume nausea means you’re allergic to opioids. It’s a side effect, not an allergy. Many people who stop opioids because of nausea end up switching to a different one and feeling fine.

Here’s a key point: nausea can also show up during withdrawal. If you suddenly stop taking opioids, you’ll likely get nausea, vomiting, and diarrhea-not because the drug stopped working, but because your body is in shock. That’s why tapering off slowly under medical care is critical. The FDA warns that abrupt discontinuation can trigger uncontrolled pain, anxiety, and even suicidal thoughts.

Why Managing Side Effects Isn’t Optional

These side effects aren’t just annoying. They’re treatment killers. If you’re too constipated to leave the house, too drowsy to work, or too nauseated to eat, you’ll stop taking your meds-even if you’re still in pain. And when you stop suddenly, withdrawal hits hard. That’s when people turn to street drugs, which are far more dangerous.

The CDC reports that over 8 million Americans misused prescription opioids in 2023. More than two-thirds of them said they did it to relieve physical pain. That’s telling. It means many people aren’t getting their pain under control with safe, managed care. They’re self-treating because the side effects were never addressed.

Good pain management isn’t just about choosing the right drug. It’s about planning for the side effects before they start. That means:

• Starting laxatives on day one
• Taking opioids at night if drowsiness is an issue
• Trying different anti-nausea meds until you find one that works
• Never stopping opioids cold turkey
• Talking to your doctor before adding any other sedating meds

There’s a myth that opioids are only for end-of-life care. That’s not true. Millions of people with chronic pain live full lives on them-when side effects are managed well. The goal isn’t to avoid opioids. It’s to use them safely, with a plan.

What to Do Next

If you’re on opioids and dealing with constipation, drowsiness, or nausea:

1. Don’t wait. Talk to your doctor today.
2. Ask if you’re on the right dose-sometimes less is more.
3. Request a bowel regimen if you’re not already on one.
4. Keep a symptom log: when nausea hits, how sleepy you feel, how often you go to the bathroom.
5. Ask about PAMORAs if laxatives aren’t enough.

There are alternatives. Physical therapy, nerve blocks, non-opioid pain relievers like gabapentin or duloxetine, and even cognitive behavioral therapy can reduce opioid needs. But if you need opioids, you deserve to take them without suffering through side effects that could’ve been prevented.