Methadone hydrochloride is available in the United States in multiple formulations, including 5-mg, 10-mg, and 40-mg scored tablets. These tablets, unlike sustained-release formulations of other opioids can be divided or crushed³; the 5- and 10-mg tablets can be administered to treat moderate to severe pain in patients who fail to respond to nonnarcotic analgesics. Methadone hydrochloride is also available in solution form in concentrations of 5 mg/5 mL, 10 mg/5 mL, and 10 mg/mL for oral administration, and a 10-mg/mL solution for parenteral administration.⁴ 

Methadone, a highly lipophilic drug, is rapidly absorbed with extensive tissue distribution.⁵ Unlike morphine sulfate, methadone has no active metabolites; hepatic metabolism has no significant effect on methadone concentrations, clearance, or clinical disposition.⁶ Renal and/or hepatic impairment does not alter clearance or dosing of methadone. It is predominantly excreted in the feces; however, acidification of the urine will increase renal excretion. It has a prolonged and variable elimination phase with a plasma half-life ranging from 4.2 hours to 190.0 hours.^5,7-9 The mean plasma half-life of methadone is probably 15 to 60 hours,⁷ though even this range is extremely variable and dependent on single versus multiple dosing, individual adipose stores, and protein binding. This wide variation in half-life contributes to a substantial possibility of toxic accumulation. 

Methadone binds with mu-, delta-, and, to a lesser extent, kappa-opioid receptor sites (Figure 1)¹⁰ and has a rapid onset of action; analgesic effects occur within 30 to 60 minutes and peak between 2.5 and 4.0 hours. Its oral bioavailability, though variable, generally exceeds 80%. 

Adverse reactions that occur during methadone administration are similar to those found with other mu-opioid agonists, including pruritus, nausea, constipation, confusion, sedation, and respiratory depression. In addition, excess sweating (diaphoresis) and flushing can occur. Because life-threatening toxicities may not become observable for 2 to 5 days after initiation of methadone therapy, caution should be taken at commencement of dosing; increases should be carefully monitored.³ Fatal or life-threatening respiratory depression and cardiac arrhythmias in methadone-naïve patients receiving oral doses continue to be reported by the US Food and Drug Administration (FDA) as recently as November 27, 2006.¹¹ Therefore, the need for appropriate attention to dosing and side effects is always indicated. 

Cytochrome P450 is the main isoenzyme involved in methadone biotransformation,⁵ yet little clinical information is known regarding its drug interactions. Because a majority of medications are metabolized via this pathway, physicians must be sensitive to co-administration of other drugs that could result in either an increase or a reduction of methadone levels (Table 1⁵). 

Although initially used in patients with cancer, methadone is being increasingly used in the end-of-life care setting for patients with nonmalignant pain syndromes. As the only long-acting opioid liquid formulation, methadone provides an attractive alternative to the expensive transdermal fentanyl patch in patients with debilitating states of advanced dementia or with arthritis, in deconditioned bedridden individuals with adult failure to thrive who have generalized pain or allodynia, and when patients can no longer swallow pills. The high bioavailability and long duration of action following rectal administration offer an alternative to intravenous administration.¹⁰ Another advantage to methadone is the fact that because it is synthetic and has no cross-allergenicity, it may be used in patients with morphine allergy.³ Additionally, the relatively slow development of action and long duration serve to reduce establishment of reward behaviors that can occur with faster-acting and shorter-duration opioids.³ 

Whereas methadone and fentanyl have been shown to be safe in patients with renal failure,⁶ morphine and codeine should be avoided because their active metabolites accumulate. Clinical data in the literature regarding the use of hydro-morphone and oxycodone with renal failure are sparse, and because both of these medications are primarily excreted in the urine and also have the potential for toxic accumulation, they should be used with caution. An additional advantage of methadone is its property as an N-methyl-d-aspartate (NMDA) receptor antagonist. This action contributes to a reduced propensity to develop opioid tolerance as compared with morphine, and a possibly greater efficacy in treating patients with neuropathic pain.^5,6,8,12 (Figure 2) 

Equianalgesic dosing of methadone is more complex than it is for other opioids. Unlike morphine, methadone exhibits wide variations in half-life among patients and must be cautiously prescribed, especially in individuals currently medicated with an opioid. 

There are several approaches to prescribing methadone. In end-of-life care where some patients have noncancer pain syndromes and debilitated elderly patients have moderate pain, a reasonable approach is to start at 2.5 mg to 5 mg every 12 hours. Additional increases are determined based on the frequency and amount of short-acting opioid used for breakthrough or incidental pain, and titrated accordingly every 5 days. Although no method of conversion has emerged as being superior, two examples of established protocols¹³ for both initiating and converting to methadone follow. 

This is the easiest method for initiating treatment with methadone in opioidnaïve patients: 

Table 2¹³ provides the conversion ratio of oral morphine to methadone. 

The process of switching from another opioid to methadone, especially when high doses are being used, is more complex. Several conversion protocols are available.^13-15 One example is: 

Example—If prior daily opioid dose equals 150 mg of oral morphine sulfate equivalent per day; then, use 15 mg of methadone hydrochloride every 3 hours as needed. (Note: This is not a 1:10 ratio unless only one dose is given in 24 hours: 1:10 ratio would be 15 mg/d, not 15 mg per dose.) 

On day 6, calculate total amount of methadone taken during previous 48 hours and convert to twice-daily methadone dose. If the patient actually took the 15 mg dose every 3 hours on days 4 and 5, then the correct dosing would be 60 mg every 12 hours. 

Example—Patient is taking 600 mg of oral morphine sulfate equivalent per day. Because the oral morphine equivalent is greater than 300 mg/d, use 30 mg of methadone hydrochloride as initial fixed dose after terminating morphine administration and give 30 mg of methadone hydrochloride every 3 hours as needed. If the patient requires eight doses of 30 mg each for a total of 240 mg days 4 and 5 (120 mg/d); then, on day 6, adjust methadone dose to 40 mg orally every 8 hours or 60 mg every 12 hours.^14,15 

Figure 3 provides a list of additional print and Web site resources. 

A 46-year-old man with head and neck cancer status post radical dissection is taking the following medications:  

The approach to pain control in this patient would be as follows: 

Total equianalgesic dose equals 1000 mg per 24 hours  

In the patient described in the case scenario, pain control was initiated with administration of methadone hydrochloride, 15 mg every 8 hours, with four 8-mg tablets of hydromorphone hydrochloride (32 mg) every 2 hours as needed for pain. The patient utilized 10 doses of hydromorphone daily for 2 days, then 5 to 8 doses per 24 hours for the next 3 days. After the fifth day, his pain was well controlled with 3 doses of hydromorphone daily.  

Methadone is gaining recognition in the arsenal of pain management. With appropriate knowledge and initial, cautious titration, physicians can readily give consideration to administration of methadone as they would to extended-release formulations of morphine, oxycodone, hydromorphone, and fentanyl. The efficacy, long-acting liquid formulations, multiple routes of administration, and low cost make methadone a noteworthy contender in treatment of patients with chronic pain.