[10] It is also available by itself in a long-acting form sold under the brand name Zohydro ER, among others, to treat severe pain of a prolonged duration.

[10] Serious side effects may include low blood pressure, seizures, QT prolongation, respiratory depression, and serotonin syndrome.

[15] Hydrocodone is believed to work by activating opioid receptors, mainly in the brain and spinal cord.

However, in a study of emergency department patients with fractures, it was found that an equal amount of either drug provided about the same degree of pain relief, indicating that there is little practical difference between them when used for that purpose.

[5][2] Common side effects of hydrocodone are nausea, vomiting, constipation, drowsiness, dizziness, lightheadedness, anxiety, abnormally happy or sad mood, dry throat, difficulty urinating, rash, itching, and contraction of the pupils.

[35] Several cases of progressive bilateral hearing loss unresponsive to steroid therapy have been described as an infrequent adverse reaction to hydrocodone/paracetamol misuse.

A newborn of a mother taking opioid medications regularly prior to the birth will be physically dependent.

[42] An epidemiological study indicated that opioid treatment during early pregnancy results in increased risk of various birth defects.

[42] Hydrocodone taken concomitantly with serotonergic medications like SSRI antidepressants may increase the risk of serotonin syndrome.

[2] The FDA label for immediate-release hydrocodone with acetaminophen does not include any information on the influence of food on its absorption or other pharmacokinetics.

[57] Conversely, coadministration with a high-fat meal increases peak concentrations of different formulations of extended-release hydrocodone by 14 to 54%, whereas area-under-the-curve levels are not notably affected.

[7][2] The hepatic cytochrome P450 enzyme CYP2D6 converts hydrocodone into hydromorphone, a more potent opioid (5-fold higher binding affinity to the MOR).

[64] However, norhydrocodone is actually a MOR agonist with similar potency to hydrocodone, but has been found to produce only minimal analgesia when administered peripherally to animals (likely due to poor blood–brain barrier and thus central nervous system penetration).

[8][9] Hydrocodone concentrations are measured in blood, plasma, and urine to seek evidence of misuse, to confirm diagnoses of poisoning, and to assist in investigations into deaths.

Blood and plasma hydrocodone concentrations typically fall into the 5–30 μg/L range among people taking the drug therapeutically, 100–200 μg/L among recreational users, and 100–1,600 μg/L in cases of acute, fatal overdosage.

Co-administration of the drug with food or alcohol can very significantly increase the resulting plasma hydrocodone concentrations that are subsequently achieved.

[71][72] Hydrocodone is most commonly synthesized from thebaine, a constituent of opium latex from the dried poppy plant.

This name is analogous to other products the company introduced or otherwise marketed: Dilaudid (hydromorphone, 1926), Dinarkon (oxycodone, 1917), Dihydrin (dihydrocodeine, 1911), and Dimorphan (dihydromorphine).

[84][85][86][87] In 2011, hydrocodone products were involved in around 100,000 abuse-related emergency department visits in the United States, more than double the number in 2004.