Is Tylenol #3 affected by genetics?

Partially — 1 of 2 ingredients has pharmacogenetic evidence.

Relevant genes: CYP2D6

Used for: Mild to moderate pain, often post-surgical or dental

Tylenol #3 (also marketed as Tylenol with Codeine) combines acetaminophen with codeine, and the codeine component is one of the cleanest examples in all of pharmacogenetics. Codeine itself is largely inactive as a painkiller. CYP2D6 in your liver converts codeine into morphine, which is what actually relieves pain. If your CYP2D6 variant makes you a poor metabolizer, very little morphine is produced and Tylenol #3 feels like plain Tylenol. If you're an ultrarapid metabolizer, the opposite happens: codeine is converted to morphine too efficiently, blood morphine levels spike, and you're at real risk of sedation, respiratory depression, and (in at least one well-documented pediatric case) death. This is why the FDA added a boxed warning against codeine use in children who might be ultrarapid metabolizers.

What's in Tylenol #3

codeine affected by CYP2D6

Affected by CYP2D6 · CPIC, FDA · Strong evidence

Codeine is a prodrug activated by CYP2D6 to morphine. About 7 percent of people of European descent are CYP2D6 poor metabolizers, meaning they get minimal analgesic effect from codeine. A smaller percentage are ultrarapid metabolizers who convert codeine to morphine much faster than expected. CPIC explicitly recommends avoiding codeine in both poor and ultrarapid metabolizers, because one group gets no benefit and the other gets dangerous exposure.

Read the full codeine genetics guide →

acetaminophen no known pharmacogenetics

No strong published pharmacogenetic guideline currently applies to acetaminophen.

Acetaminophen is cleared by UGT and sulfate conjugation and does not have a clinically actionable pharmacogenetic guideline. In Tylenol #3, acetaminophen provides its own baseline analgesic effect, which is why even CYP2D6 poor metabolizers get some pain relief from the combination, just not the additional opioid-mediated relief.

Codeine phenotype recommendations

Published guidance from CPIC and FDA on how codeine should be dosed or substituted based on your CYP2D6 phenotype.

Phenotype	What it means	Recommendation	Evidence
Ultrarapid Metabolizer CYP2D6	Your body converts codeine to morphine too quickly, raising the risk of dangerous side effects or overdose.	CPIC Avoid codeine use because of potential for serious toxicity. If opioid use is warranted, consider a non-tramadol opioid. FDA Avoid codeine. Use an alternative pain medication. The risk of life-threatening side effects is significantly increased.	Strong
Normal Metabolizer CYP2D6	Your body processes codeine normally; the usual recommended dose is likely to work well for you.	CPIC Use codeine label recommended age- or weight-specific dosing. FDA Initiate therapy with recommended starting dose.	Strong
Intermediate Metabolizer CYP2D6	Your body processes codeine more slowly, which might reduce its pain-relieving effect. If it doesn't work, another medication may be needed.	CPIC Use codeine label recommended age- or weight-specific dosing. If no response and opioid use is warranted, consider a non-tramadol opioid. FDA Initiate therapy with recommended starting dose.	Moderate
Poor Metabolizer CYP2D6	Your body doesn’t effectively convert codeine into its active pain-relieving form, so it likely won’t provide enough pain relief.	CPIC Avoid codeine use because of possibility of diminished analgesia. If opioid use is warranted, consider a non-tramadol opioid. FDA Consider an alternative pain medication. Codeine may not provide adequate pain relief.	Strong
Indeterminate CYP2D6	The impact of your genotype on response to this drug is unknown.	CPIC + FDA Initiate therapy with recommended starting dose.	—
Not available CYP2D6	The impact of your genotype on response to this drug is unknown.	CPIC + FDA Initiate therapy with recommended starting dose.	—

Sources: CPIC, FDA

The gene behind the guidance

CYP2D6 Cytochrome P450 2D6

CYP2D6 is the most clinically important pharmacogene. It metabolizes around a quarter of all prescription drugs, including many antidepressants, opioids, and stimulants. The gene is unusually variable: roughly 7 percent of people are poor metabolizers (they barely activate CYP2D6), and another 1 to 3 percent are ultrarapid metabolizers (their enzyme is overactive).

For most CYP2D6 drugs, poor metabolizers feel stronger effects and more side effects at standard doses, while ultrarapid metabolizers may feel almost nothing. For prodrugs like codeine, the relationship flips: poor metabolizers feel less effect because they can't activate the drug.

See all drugs affected by CYP2D6 →

Browse the full drug-class: Pain medications.

What this means for you

Tylenol #3 is one of the few medications where knowing your CYP2D6 phenotype is legitimately safety-relevant before taking it. If you're a poor metabolizer and Tylenol #3 hasn't worked for you after a procedure, it's not in your head: the drug isn't activating. Ask for an alternative like morphine directly, hydromorphone, or a non-opioid strategy. If you're an ultrarapid metabolizer, avoid codeine entirely, and flag it on your medical history so future prescribers know.

FAQ

Why would I not feel Tylenol #3?

The most common non-tolerance explanation is that you're a CYP2D6 poor metabolizer. Without functional CYP2D6, codeine can't convert to morphine, and Tylenol #3 feels like a regular Tylenol dose plus a placebo. About 7 percent of people of European descent are in this category, with higher rates in some other populations.

Is there a test for whether codeine will work for me?

Yes. CYP2D6 genotyping is widely available and is one of the longest-established pharmacogenetic tests. The CPIC guideline for codeine is considered strong evidence, and many hospitals test before prescribing opioids for children specifically because of codeine's pharmacogenetic profile.

What should I use if I can't take codeine?

Your prescriber has several non-codeine options. Morphine and hydromorphone don't depend on CYP2D6 activation (they're active as given). Tramadol also depends on CYP2D6, so it isn't a solution. Non-opioid strategies like NSAIDs plus scheduled acetaminophen can work well for many moderate pain scenarios. Talk to your prescriber rather than self-selecting.

Related medications

Find out how your genetics affect Tylenol #3

This page describes the general pharmacogenetics. A Gene2Rx report analyzes your own DNA to tell you which metabolizer group you fall into, across every medication.

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Informational only — not medical advice. Pharmacogenetic guidance describes population-level patterns; your individual response depends on many factors. Never start, stop, or change a medication without talking to your prescribing clinician.