Codeine: Complete Guide to Uses, Dosage, Side Effects & Risks

Codeine is one of the oldest and most historically significant opioids in medicine. A natural alkaloid extracted from opium, codeine has been used for over 150 years to treat pain, suppress cough, and relieve diarrhea. Today it remains among the most prescribed opioids worldwide — often found in combination products paired with acetaminophen for pain relief or with promethazine as a prescription cough syrup.

Despite its long history and relatively mild reputation compared to stronger opioids, codeine is a drug that demands respect. It carries real risks of addiction, life-threatening respiratory depression, and — critically — unpredictable toxicity in individuals with a genetic variation that causes ultra-rapid conversion to morphine. Understanding codeine means understanding both its medical value and its significant potential for harm.

What Is Codeine?

Codeine (3-methylmorphine) is a naturally occurring opioid alkaloid found in the resin of the opium poppy Papaver somniferum. Typically comprising about 0.2–3% of raw opium by weight, codeine can be extracted directly from opium or produced semi-synthetically by methylation of morphine. It was first isolated in 1832 by French chemist Pierre Robiquet and has been used medicinally ever since.

In the United States, codeine has a complex scheduling status reflecting its different formulations and concentrations:

• Schedule II — pure codeine preparations and single-entity codeine products (codeine sulfate tablets); high potential for abuse
• Schedule III — combination products with ≤90 mg codeine per dosage unit paired with a non-narcotic (e.g., Tylenol with Codeine #3 and #4 — codeine 30 mg or 60 mg with acetaminophen 300 mg)
• Schedule V — cough preparations containing ≤200 mg codeine per 100 mL or per 100 g (historically available OTC in some states, but now prescription-only in all US states)

How Does Codeine Work?

Codeine itself has very low direct affinity for opioid receptors — it is what pharmacologists call a prodrug. Approximately 5–10% of a codeine dose is converted in the liver by the CYP2D6 enzyme to morphine, which is the active compound responsible for codeine's analgesic and antitussive effects. The remaining codeine is metabolized to codeine-6-glucuronide (with weak opioid activity) and norcodeine (inactive).

This prodrug nature has a critical clinical implication: patients who lack functional CYP2D6 (poor metabolizers, about 5–10% of the population) produce little or no morphine and get virtually no benefit from codeine. Conversely, patients who are CYP2D6 ultra-rapid metabolizers (1–7% of the population) convert codeine to morphine at an accelerated rate, producing dangerously high morphine levels even at standard doses — leading to respiratory depression and, in documented cases, death.

Beyond opioid receptor activity, codeine is one of the most effective antitussives (cough suppressants) known, acting on the cough center in the medulla oblongata to raise the threshold at which coughing is triggered. This makes codeine-containing cough syrups highly effective for dry, non-productive cough when short-term relief is needed.

Common Brand Names and Products Containing Codeine

• Tylenol with Codeine #3 — acetaminophen 300 mg + codeine 30 mg; among the most commonly prescribed combination products
• Tylenol with Codeine #4 — acetaminophen 300 mg + codeine 60 mg; for more severe pain
• Fiorinal with Codeine — aspirin, butalbital, caffeine, and codeine; for tension headache with pain
• Promethazine with Codeine cough syrup — prescription cough syrup; codeine 10 mg/5 mL + promethazine 6.25 mg/5 mL; widely known as the basis of "lean" or "purple drank"
• Robitussin AC — codeine-guaifenesin cough syrup; prescription-only
• Codeine Phosphate tablets — available as 15 mg, 30 mg, 60 mg; Schedule II

Medical Uses of Codeine

Codeine has two primary approved indications in adults:

Pain Relief

Codeine is indicated for the relief of mild to moderately severe pain. It is frequently prescribed for dental extractions, minor orthopedic injuries, and postoperative pain following outpatient procedures — situations where ibuprofen or acetaminophen alone may be insufficient but a more potent Schedule II opioid like hydrocodone is not necessary. It is commonly combined with acetaminophen (Tylenol with Codeine) to enhance analgesia through synergistic mechanisms.

Cough Suppression

Codeine is one of the most effective antitussives available. It is indicated for the symptomatic relief of non-productive (dry) cough when other treatments have proven inadequate. Codeine suppresses the cough reflex more effectively than most OTC antitussives including dextromethorphan, particularly for severe or persistent cough related to post-viral irritation, tracheitis, or bronchitis.

Off-Label Uses

Codeine is occasionally used off-label for diarrhea (it reduces intestinal motility, the same mechanism exploited in loperamide), and historically was used as a component of compound cough preparations. These uses are less common today given the availability of safer alternatives.

Codeine Dosage

Dosing for codeine varies by indication and formulation. Standard adult dosing guidelines:

The CYP2D6 Genetic Problem: Why Codeine Can Be Unpredictably Dangerous

The most important safety concept with codeine is its dependence on CYP2D6 genetics for conversion to active morphine. This creates two opposite problems:

Ultra-Rapid Metabolizers (CYP2D6 UM)

Ultra-rapid metabolizers — about 1–7% of people, with higher rates in North African and Middle Eastern populations (up to 29% in some Ethiopian populations) — convert codeine to morphine far faster than normal. Even standard doses produce toxic morphine levels in these individuals. The FDA's 2013 black box warning came after documenting deaths in pediatric patients who were ultra-rapid metabolizers and received codeine for post-tonsillectomy pain. Nursing infants whose mothers were ultra-rapid metabolizers also died from morphine toxicity in breast milk.

Poor Metabolizers (CYP2D6 PM)

Poor metabolizers — about 5–10% of people — produce little or no morphine from codeine. For these patients, codeine is essentially ineffective as a pain reliever. If a patient reports that codeine "does nothing" for their pain, this is likely the explanation. Prescribing higher doses in an attempt to compensate simply increases side effects without improving analgesia.

Pharmacogenomic testing can identify CYP2D6 status, and some institutions now use this testing to guide opioid prescribing. However, it is not yet standard of care in all settings.

Side Effects of Codeine

Common Side Effects

• Constipation — nearly universal; begin bowel regimen with prolonged use
• Nausea and vomiting — especially when initiating treatment
• Drowsiness and sedation
• Dizziness and lightheadedness
• Dry mouth
• Itching (pruritus)
• Headache
• Stomach pain

Serious Side Effects

• Respiratory depression — particularly dangerous in ultra-rapid metabolizers, elderly patients, and those with sleep apnea
• Addiction and dependence — with regular use, even at therapeutic doses
• Liver injury — from the acetaminophen component in combination products; do not exceed 4,000 mg acetaminophen per day from all sources
• Allergic reactions — including anaphylaxis; patients with true morphine allergy may also react to codeine

Codeine Misuse: "Lean" and Cough Syrup Abuse

Codeine-promethazine cough syrup (prescription-strength) has become a widely recognized drug of abuse, particularly in the form of "lean," "purple drank," or "sizzurp." This involves mixing the cough syrup with soft drinks such as Sprite or Mountain Dew, sometimes with hard candy added. The combination of codeine's opioid effects and promethazine's sedating antihistamine effects produces euphoria, sedation, and slowed movement that has been glamorized in rap and hip-hop culture.

Lean abuse is extremely dangerous. The codeine provides opioid-mediated euphoria and respiratory depression, while the promethazine adds anticholinergic and antihistamine effects. At abused doses — often far above therapeutic levels — lean causes profound respiratory depression, cardiovascular depression, seizures, and death. Multiple high-profile deaths have been linked to lean abuse.

Codeine Addiction and Withdrawal

Despite its reputation as a "mild" opioid, codeine produces real physical dependence with regular use. Withdrawal symptoms, mediated by morphine withdrawal (since morphine is codeine's active metabolite), include anxiety, restlessness, muscle aching, sweating, chills, nausea, vomiting, diarrhea, insomnia, and intense drug craving. These symptoms typically begin within 8 to 24 hours of the last dose and peak at 36 to 72 hours.

Codeine dependence can be treated with medically supervised tapering of the dose, or — in cases of significant addiction — with buprenorphine/naloxone (Suboxone) treatment. Behavioral therapy is an important component of addiction recovery. Naltrexone can be used for relapse prevention after detoxification.

Drug Interactions With Codeine

• Alcohol — potentiates CNS and respiratory depression; potentially fatal
• Benzodiazepines (Xanax, Valium, Klonopin) — FDA black box warning; dramatically increases overdose death risk
• Other CNS depressants — sleep aids, antihistamines, muscle relaxants: additive sedation and respiratory depression
• MAO inhibitors — contraindicated; risk of severe CNS excitation, hyperthermia, and hypotension
• CYP2D6 inhibitors (fluoxetine, paroxetine, bupropion, quinidine) — block conversion to morphine; may reduce efficacy or cause atypical toxicity
• CYP3A4 inhibitors (ketoconazole, clarithromycin, erythromycin) — increase codeine and metabolite levels; increase toxicity risk
• Anticholinergic drugs — additive constipation and urinary retention
• Acetaminophen (other sources) — risk of hepatotoxicity when combined with combination codeine/APAP products

Overdose Recognition and Response

Codeine overdose mimics morphine overdose since morphine is the active compound. Signs include extreme drowsiness or unconsciousness, very slow or stopped breathing, pinpoint pupils, blue or gray lips, cold and clammy skin, and gurgling or choking. Call 911 immediately. Administer naloxone (Narcan) if available — available without prescription at most US pharmacies. Repeat every 2–3 minutes if no improvement. Keep the person in the recovery position and stay until EMS arrives.