Codeine is one of the weakest opioid painkillers available. It sits on the bottom rung of opioid strength, classified by the World Health Organization as a “weak opioid” suitable for mild to moderate pain. To put a number on it: codeine is roughly one-sixth the strength of morphine, milligram for milligram. That means 60 mg of codeine provides about the same pain relief as 10 mg of oral morphine.
How Codeine Compares to Other Painkillers
The standard way to measure opioid strength is to compare everything against morphine. Codeine has a conversion factor of 0.15, meaning each milligram of codeine equals just 0.15 mg of morphine. By contrast, hydrocodone and oxycodone are both several times stronger than codeine per milligram.
A clinical trial comparing codeine to hydrocodone in acute musculoskeletal pain illustrates the gap well. Patients received either 30 mg of codeine or 5 mg of hydrocodone (both combined with the same amount of acetaminophen). While average pain scores were similar between the two groups, zero patients in the hydrocodone group reported inadequate pain relief, compared to six patients in the codeine group. The takeaway: codeine works, but it’s noticeably less reliable than stronger opioids at controlling pain.
On the WHO’s three-step pain ladder, codeine occupies step two, above over-the-counter options like ibuprofen and acetaminophen but below stronger opioids like oxycodone and morphine. It’s often combined with acetaminophen or ibuprofen rather than used alone, because the combination produces better pain relief than either drug on its own.
Why Codeine Hits Differently for Different People
Here’s the thing that makes codeine unusual among painkillers: it’s essentially a prodrug. Codeine itself has very little ability to block pain. Your liver has to convert it into morphine using a specific enzyme before it does much of anything. How well your body performs that conversion varies enormously based on your genetics.
Most people convert a moderate amount of codeine into morphine and get the expected level of pain relief. But roughly 1 to 2 percent of patients are “ultra-rapid metabolizers,” meaning their bodies convert codeine into morphine much faster and more completely than normal. For these individuals, a standard dose can flood the system with more morphine than intended, raising the risk of serious side effects like dangerously slowed breathing. The prevalence of ultra-rapid metabolism varies by ethnicity: it’s around 1% in people of Chinese, Japanese, or Hispanic descent, about 3% in African Americans, 1 to 10% in Caucasians, and as high as 16 to 28% in North African, Ethiopian, and Arab populations.
On the other end of the spectrum, some people are “poor metabolizers” who barely convert codeine to morphine at all. For them, codeine provides little to no pain relief regardless of the dose.
How Long the Effects Last
Codeine kicks in within 15 to 30 minutes of taking it orally. It reaches peak effect relatively quickly, within about 30 minutes to an hour, and then tapers off over the next several hours. Total duration of pain relief is typically 4 to 6 hours per dose. The standard adult dose ranges from 15 mg to 60 mg, taken up to every four hours as needed, with a ceiling of 360 mg in a 24-hour period.
The dose-response relationship with codeine isn’t perfectly linear. One study found that 1,000 mg of acetaminophen combined with 60 mg of codeine provided nearly double the pain relief of 500 mg of acetaminophen with 30 mg of codeine. But doubling down doesn’t always help: another study found that ibuprofen 400 mg with codeine 30 mg actually provided more than double the relief of ibuprofen 800 mg with codeine 60 mg at the five-hour mark. More codeine doesn’t automatically mean more relief.
Codeine for Cough vs. Pain
Codeine has long been used for two separate purposes: pain relief and cough suppression. The doses differ. For cough, doses as low as 20 mg have been tested, though 30 to 60 mg is more common in clinical trials. For pain, the effective range starts around 30 mg and goes up to 60 mg per dose. That said, the evidence that codeine actually works well for cough suppression is surprisingly thin. No systematic review has convincingly shown that over-the-counter codeine cough preparations outperform non-opioid alternatives.
Common Side Effects
Even at its relatively low potency, codeine produces the same side effects you’d expect from any opioid. Constipation is the most persistent one and doesn’t tend to improve over time the way other side effects do. Drowsiness and nausea are common, especially in the first few days. Lightheadedness, dizziness, and a general foggy feeling round out the typical experience. These effects are generally milder than what you’d see with stronger opioids, but they’re still enough to impair driving and concentration.
Who Should Not Take Codeine
Codeine is contraindicated in children under 12 years old. It’s also prohibited for anyone under 18 recovering from tonsil or adenoid surgery, because the combination of post-surgical airway swelling and unpredictable codeine metabolism has caused fatal breathing complications in children. Adolescents between 12 and 18 who have obesity, obstructive sleep apnea, or lung disease face elevated risk as well, because all of these conditions make the respiratory depression caused by opioids more dangerous.
The genetic variability in codeine metabolism is the core safety concern. Unlike stronger opioids that act directly, codeine’s reliance on liver conversion means its actual potency in your body is partly a genetic lottery. Two people can take the same 30 mg tablet and end up with very different amounts of morphine in their bloodstream. This unpredictability is a major reason why many countries have moved codeine behind the pharmacy counter or restricted it to prescription-only status.