The M523 pill is a white oblong tablet containing oxycodone 10mg/acetaminophen 325mg — the generic form of Percocet 10/325, a Schedule II controlled substance manufactured by Mallinckrodt. Oxycodone is more potent than hydrocodone milligram-for-milligram, and the M523 imprint has been documented in confirmed counterfeit tablets containing fentanyl, meaning any pill bearing this description obtained outside a licensed pharmacy must be treated as potentially lethal.
What Is the M523 Pill? Oxycodone/Acetaminophen 10/325mg
A white oblong tablet imprinted with "M523" on one side and "10/325" on the other is oxycodone hydrochloride 10mg combined with acetaminophen 325mg — the generic formulation of Percocet at its standard full-strength dose, manufactured by Mallinckrodt Pharmaceuticals. This combination analgesic belongs to Schedule II of the Controlled Substances Act, the most restrictive classification assigned to drugs with accepted medical use. If you've found this pill or been prescribed it, understanding both its therapeutic role and its substantial risks is not optional — it's essential.
COUNTERFEIT ALERT: Pills pressed with the M523 imprint and matching description have been documented as counterfeits containing fentanyl. Pills obtained outside a licensed pharmacy with a valid prescription must be assumed potentially lethal. Visual inspection cannot distinguish a real M523 from a fake. Never take a pill of this description from an unverified source.
What Oxycodone Is
Oxycodone is a semi-synthetic opioid derived from thebaine, an alkaloid found in the opium poppy. Like all opioids, it works by binding to mu-opioid receptors throughout the brain, spinal cord, and peripheral nervous system. Activation of these receptors inhibits pain signal transmission, reduces the emotional distress associated with pain, and — at higher doses or in susceptible individuals — produces euphoria through reward circuitry engagement in the limbic system. Oxycodone is considered more potent than hydrocodone on a milligram-per-milligram basis, though direct comparisons depend on formulation and individual metabolism.
In practice, a 10mg oxycodone dose is meaningfully more potent than a 5mg hydrocodone dose (found in tablets like M365). For someone with no opioid tolerance, 10mg oxycodone represents a substantial analgesic dose that carries real respiratory depression risk if dosing instructions are not followed, or if the medication is combined with other CNS depressants.
Medical Uses
Oxycodone/acetaminophen combination tablets are indicated for moderate to moderately severe pain where a combination analgesic is appropriate. Common clinical contexts include:
- Post-surgical pain management in the immediate recovery period
- Severe acute injury — fractures, major trauma, significant musculoskeletal damage
- Cancer pain when moderate opioid therapy is appropriate
- Dental pain following complex oral surgery (though prescribing patterns have shifted toward lower-dose options)
Prescribing guidelines from major medical organizations have increasingly emphasized limiting opioid prescriptions to the shortest effective duration, generally recommending that acute pain prescriptions not exceed three to seven days for most non-cancer indications.
The Acetaminophen Component: A Risk You Can't Ignore
Many people focus exclusively on oxycodone when thinking about M523's risks, but acetaminophen demands equal attention. Acetaminophen is safe at recommended doses — the maximum daily intake for adults is 4,000mg, with 3,000mg recommended as a more conservative ceiling for regular use. The danger arises when people take multiple products containing acetaminophen simultaneously without recognizing the overlap.
Acetaminophen is hidden in dozens of combination products: NyQuil, Theraflu, Excedrin, many prescription pain combinations, and many others. A patient taking M523 every six hours is already consuming 1,300mg of acetaminophen daily from this medication alone. Adding even a single dose of Tylenol or a cold medicine containing acetaminophen can push total intake to dangerous levels. Acetaminophen overdose is one of the leading causes of acute liver failure in the United States — and the liver damage it causes is not always accompanied by early warning symptoms.
The 325mg of acetaminophen in M523 reflects FDA guidance issued in 2014 limiting prescription combination products to no more than 325mg acetaminophen per dose — a reduction from the previous 500mg standard — specifically to reduce unintentional overdose. Always read labels of every OTC medication you take concurrently and alert your physician or pharmacist.
Signs of Opioid Overdose
Opioid overdose is a medical emergency. Knowing the signs can save a life — whether it's the person prescribed M523 who took too much, or someone who found the medication without context. The classic overdose triad is:
- Pinpoint (very small) pupils — even in low light conditions
- Unconsciousness or extreme unresponsiveness — cannot be woken by voice or sternal rub
- Slow, shallow, or stopped breathing — fewer than 12 breaths per minute, or breathing with long pauses between breaths
Additional signs include blue or grayish lips and fingertips (cyanosis from oxygen deprivation), gurgling or choking sounds, and limpness. If you observe these signs, call 911 immediately. If naloxone (Narcan) is available, administer it — it is safe to give even if you are not certain the cause is opioids, and it can reverse respiratory depression rapidly. Lay the person in the recovery position (on their side) to prevent aspiration if they vomit.
Naloxone nasal spray (Narcan, Kloxxado) is available without a prescription at most U.S. pharmacies. Households where opioids are prescribed should keep it on hand. Its half-life is shorter than oxycodone's, meaning repeat doses may be needed if the person re-sedates before emergency services arrive.
Physical Dependence and Addiction Risk
Oxycodone is one of the drugs at the center of the opioid epidemic, and Schedule II classification reflects its high addiction potential. Physical dependence — meaning the body adapts to the presence of the drug and experiences withdrawal when it's removed — can develop within a week or two of around-the-clock use. Withdrawal from opioids, while intensely uncomfortable (flu-like symptoms, severe anxiety, insomnia, muscle cramping, GI distress), is not typically fatal in otherwise healthy adults, distinguishing it from benzodiazepine withdrawal. However, relapse risk during withdrawal is high, and relapse after a period of abstinence — when tolerance has dropped — is a common context for fatal overdose.
STORAGE AND DISPOSAL: Store M523 in a secure location away from children and adolescents. Opioid medications left in medicine cabinets are a primary source of diversion and first exposure for young people. When your prescription is complete, dispose of remaining tablets at a DEA-authorized medication take-back location (findtreatment.gov/locator lists sites nationwide) or use an FDA-approved drug deactivation pouch. Do not flush oxycodone/acetaminophen tablets unless no take-back option is available — the FDA permits flushing of certain opioids as a safety measure of last resort.
Frequently Asked Questions
What is the M523 pill?
The M523 pill is a white oblong tablet containing oxycodone 10mg and acetaminophen 325mg — the generic equivalent of Percocet 10/325 — manufactured by Mallinckrodt Pharmaceuticals. It is a Schedule II controlled substance prescribed for moderate to severe pain. The "M" in the imprint is Mallinckrodt's manufacturer identifier.
How does M523 differ from M367?
Both are white oblong opioid/acetaminophen combination tablets from Mallinckrodt, but they contain different opioids: M523 contains oxycodone 10mg, while M367 contains hydrocodone 10mg. Oxycodone is generally considered more potent than hydrocodone on a milligram-for-milligram basis. Both contain the same 325mg of acetaminophen and carry similar risks regarding acetaminophen accumulation from other sources.
Is there a risk of counterfeit M523 pills?
Yes. Counterfeit tablets bearing the M523 imprint — visually indistinguishable from the genuine pharmaceutical — have been documented in DEA analyses to contain illicitly manufactured fentanyl. This represents an extreme hazard: fentanyl is far more potent than oxycodone, and a tablet containing even a modest amount of fentanyl in place of oxycodone can be fatal. Only pills dispensed from a licensed pharmacy under a valid prescription can be considered safe.
What should I do if I'm not sure an M523 tablet is genuine?
If you have any doubt about a tablet's authenticity — if it wasn't dispensed to you directly from a licensed pharmacy — do not take it. Contact your pharmacist, who can confirm legitimate medication sources and provide verified medication. Fentanyl test strips can detect fentanyl contamination but are not 100% reliable and do not rule out other dangerous adulterants. When in doubt, discard via a DEA drug take-back program.
Verify the M523 imprint using our free pill identification tool
Identify This Pill