Methadone is generally used for medical purposes because it lasts longer than traditional opiates such as morphine. It is also used as a treatment for opiate addiction, for both those reasons and the fact that it does not result in a euphoric "rush" after administration like many traditional opiates.
Previous opiate addiction treatments involved using decreasing treatments of opiates such as heroin or morphine. Many people derided such programs; characterizing them as simply providing addicts with their fix. Methadone treatment was introduced both for the aforementioned long-lasting and non-euphoric effects, and to combat this perception. But, we're still treating addiction to a substance with the very type of substance the people are addicted to in the first place! Why is that?
As you might be aware, the human body is full of various neurotransmitters, which have corresponding receptors. Opioids, such as methadone, are a form of neurotransmitter, therefore it follows that the body produces several varieties of them and has receptors to greet them as well. Another common source are derivatives of the poppy plant, properly called opiates, though many refer to all opioids as opiates, like morphine and heroin (the original brand name for diacetylmorphine, a medication sold by Bayer in the early 20th century).
Opioids are also well known for their ability to induce euphoria. Combined with the relative ease in which a tolerance to them is built, it is common for many to become addicted to them. Continued use of externally-provided opioids in increasing doses causes the human body to reduce or stop production of its aforementioned endogenous opioids, further compounding the problem. When an individual wishes to eliminate their dependency on opioids, or is compelled to by authorities, it is necessary to slowly reduce dosage over time, to give the human body ample opportunity to adjust to the changes and resume or ramp up production of its own endogenous opioids. Such treatment also reduces the pain and suffering associated with opiate withdrawal. For these reasons, for many years now doctors have used several different kinds of opioids to treat opioid addiction, most recently methadone.
Of course, as the use of methadone in treating pain and addiction rose, so did the number of deaths caused by it. In 2002, the Substance Abuse and Mental Health Services Administration (SAMHSA) of the United States Department of Health & Human Services reported on the increasing number causes of methadone deaths:
Hospital emergency department visits involving methadone rose 176 percent from 1995 to 2002. The rise from 2000 to 2002 was 50 percent, according to SAMHSA’s Drug Abuse Warning Network.
The experts surmise that current reports of methadone deaths involve one of three scenarios: illicitly obtained methadone used in excessive or repetitive doses in an attempt to achieve euphoric effects; methadone, either licitly or illicitly obtained, used in combination with other prescription medications, such as benzodiazepines (anti-anxiety medications), alcohol or other opioids; or an accumulation of methadone to harmful serum levels in the first few days of treatment for addiction or pain, before tolerance is developed.
In 2006, the Food & Drug Administration (FDA) issued an alert to medical professionals advising them of the increasing number of methadone related deaths and providing guidelines to reduce such deaths in the future. Nonetheless, methadone use and death rates continued to rise. In 2009, SAMHSA released another report regarding methadone use and related deaths (PDF). The report indicates that deaths as a result from methadone rose from 786 in 1999 to 4,462 in 2005. It also has this to say regarding the increased use of methadone which, as jwenting points out in the comments, makes the increased number of deaths less significant:
The amount of methadone distributed or delivered by the manufacturers rose dramatically from 2000 to early 2007, with increases ranging from 9 to 22 percent annually. The distribution of all formulations of methadone (liquid, tablet, or dispersible tablet) increased. However, tablets distributed with a prescription through pharmacies had the largest increase. The number of methadone prescriptions dispensed increased by nearly 700 percent between 1998 and 2006. The strongest formulation, the 40 mg strength dispersible tablet, had the largest increase of all methadone formulations.
The report continues to put the blame for most deaths on illicit use, commonly combined with other drugs, along with other factors previously mentioned such as inappropriate prescription by doctors. Nevertheless, the report concludes that "Methadone is safe when used appropriately. It can be life saving for individuals dependent on opioids."
More recently, reports of increasing methadone deaths continue in several states, such as Oklahoma, Kentucky, and Georgia, with government officials and scientists continuing on blame deaths to illicit use and improper prescribing practices.
However, those reports also indicate that other opioids continue to be just as dangerous. While the number of methadone-related deaths has surpassed heroin, it remains lower than other prescription opiates such as oxycodone. Given the benefits of methadone treatment over other opiate treatments, I don't think we have enough information at this time to decide that it really helps more than it hurts. We could stand to find something better, but that's true for just about every medication in use today.