Zilpaterol is an agonist of β2-adrenergic receptor. Zilpaterol exerts bronchospasmolytic action. Zilmax® (4.8% Zilpaterol hydrochloride) is used to increase rate of bodyweight gain, improve feed efficiency, and increase carcass leanness in cattle fed in confinement for a period of 20-40 consecutive days at the end of the feeding period before slaughter.

Xylazine was developed as an antihypertensive agent. During clinical studies in people xylazine was found to have excessive central nervous system depressant effects and it was subsequently introduced for veterinary use as a sedative, analgesic and relaxant. Xylazine is a potent alpha-2 adrenergic agonist. Xylazine in horses and Cervidae may occasionally cause slight muscle tremors, bradycardia with partial A-V heart block and a reduced respiratory rate. Movement in response to sharp auditory stimuli may be observed.

Levomethadyl acetate (LAAM) is a synthetic opioid agonist with actions qualitatively similar to morphine (a prototypic mu agonist) and affecting the central nervous system (CNS) and smooth muscle. Principal actions include analgesia and sedation. Tolerance to these effects develops with repeated use. An abstinence syndrome generally occurs upon cessation of chronic administration similar to that observed with other opiates, but with slower onset, more prolonged course, and less severe symptoms. LAAM exerts its clinical effects in the treatment of opiate abuse through two mechanisms. First, LAAM cross-substitutes for opiates of the morphinetype, suppressing symptoms of withdrawal in opiate-dependent individuals. Second, chronic oral administration of LAAM can produce sufficient tolerance to block the subjective “high” of usual doses of parenterally administered opiates. Since the introduction of levomethadyl in 1995, the manufacturer has received increasing reports of severe cardiac-related adverse events, including QT interval prolongation, Torsades de Pointes and cardiac arrest. Other cardiac-related adverse events have also been reported, including arrhythmias, syncope, and angina. These events led to the removal of levomethadyl from the European market in March 2001. A very small number of patients may benefit from levomethadyl, but the risk of continued distribution and use no longer outweighs the overall benefits.

Meptazinol is a unique opioid analgesic. Binding studies suggest a relative selectivity for mu-1 opioid receptor sites. Meptid is indicated for the treatment of moderate to severe pain, including post-operative pain, obstetric pain and the pain of renal colic. The most commonly reported adverse reactions after treatment with meptazinol are nausea, vomiting, dizziness, diarrhoea and increased sweating, constipation, abdominal pain, rash, vertigo, headache, drowsiness, somnolence and dyspepsia.

Levorphanol Sulfate (brand name Levo-Dromoran) is an opioid medication used to treat moderate to severe pain. It is one of two enantiomers of the compound racemorphan and was first described in Germany in 1948 and approved for use in the United States in 1953 as an orally active, morphine-like analgesic. Levorphanol is approved for use in moderate to severe pain where an opioid analgesic is appropriate. Levorphanol has a wide range of activities including mu-opioid agonism, delta agonism, kappa1 and kappa3 receptor agonism, N-methyl-D-aspartate receptor antagonism and reuptake inhibition of both norepinephrine and serotonin. This multimodal profile might prove effective for pain syndromes that are refractory to other opioid analgesics, such as central and neuropathic pain and opioid-induced hyperalgesia. Levorphanol is well suited as a first-line opioid and can also be used during opioid rotation. It has no known effect on the cardiac QT interval or drug-drug interactions involving hepatic cytochrome P450s enzymes. In these regards, levorphanol may offer a superior safety profile over methadone and other long-acting opioids. Despite its prospective value of multiple mechanisms of action and the potential for treating various types of pain, levorphanol use has been largely supplanted by other recently approved opioids. Its waning use over the years has caused it to be referred to as the “Forgotten Opioid” and resulted in what some consider its underutilization. In fact, levorphanol is relatively unfamiliar to most prescribers.

Lefetamine (L-SPA; L-1,2-diphenyl-l-dimethylaminoethane hydrochloride) is a synthetic compound with analgesic and anti-inflammatory action, introduced in clinical practice in Italy and Japan as ‘Santenol’. Santenol is available for oral (50 mg tablets1 and intramuscular (60 mg vials containing also lidocainel use. Animal studies have shown an analgesic effect and changes in EEG activity and O2 consumption of the nervous tissue. Lefetamine may be an opioid partial agonist. Lefetamine was first marketed in the 1940s as an opioid analgesic. Since withdrawal symptoms were observed during treatment, it became a controlled substance.

Dextromoramide is a synthetic strong-acting opioid and full mu-opioid receptor agonist. Dextromoramide is a Schedule I drug illegal to possess. The current indication for Palfium® (dextromoramide) is severe acute or chronic pain requiring opioids, such as post-operative pain, and pain associated with bone fractures, malignancies and acute renal/biliary colic attacks in adults.

Eptazocine is an opioid analgesic which was introduced in 1987 by Morishita in Japan . It acts as a mixed κappa opioid receptor agonist and mu-opioid receptor antagonist.

Samidorphan, which was developed by Alkermes, is an opioid receptor antagonist that has been co-formulated with olanzapine into a single-dose oral tablet to mitigate the risk of weight gain while providing the therapeutic effect of olanzapine. In June 2021, the Food and Drug Administration (FDA) approved Lybalvi (olanzapine/samidorphan) for indications including treatment of adults with schizophrenia and/or bipolar I disorder (acute manic episodes or acute episodes with mixed features).