Methadone Side Effects

Doctors prescribe methadone to treat severe pain, ease severe withdrawal symptoms during opioid detoxification and as a temporary replacement for opioids. Methadone is the most well known medication used in drug replacement therapy.

German scientists first synthesized methadone in 1939. The United States Public Health Service hospitals began administering methadone as part of an opioid abstinence program by the 1950s.

While the number of opioid poisoning deaths has been on the rise for several years, methadone has had the largest relative increase. The Centers for Disease Control and Prevention, or CDC, notes the number of methadone-related poisoning deaths in the United States skyrocketed 468 percent between 1999 and 2005.

One dose of methadone lasts about 24 hours. Methadone comes as a syrup, injection or tablet. Treatment programs typically administer methadone syrup mixed in fruit juice.

Side Effects

Methadone, like all medicine, has the potential to cause side effects. The effects vary from person to person, depending on the individual's height, weight, overall health and his tolerance to opioids.

Many people experience no, or minor, side effects while using methadone. Most side effects are not serious and disappear after a few days. A few side effects are serious and require the attention of a medical professional.

The most serious side effect is respiratory problems potentially leading to stopped breathing, circulatory depression, dangerously low blood pressure and shock. Physicians should expect side effects and treat patients accordingly.

Respiratory depression is a condition where the lungs do not adequately exchange oxygen and carbon dioxide, characterized by slow or shallow breathing.

Side effects may also vary with dosage. A person whose dose is too low might experience flu-like symptoms, including runny nose, nausea or vomiting, muscle spasm and aching, trouble sleeping, anxiety and cravings. A patient whose dose is too high might suffer adverse reactions such as drowsiness, shallow breathing, pinpoint pupils, slow pulse or low blood pressure, dizziness and heart palpitations.

Allergic Reactions

Do not take methadone if you are allergic to it or other opioids. An allergic reaction is a serious medical condition that may unexpectedly turn into anaphylaxis, a life threatening condition.

Symptoms of an allergic reaction include rash, hives, itching, difficulty breathing or tightness in the chest, swelling of the mouth, face, lips or tongue.

Abuse, Dependence and Withdrawal

Potential for abuse, physical dependence or addiction are possible side effects from taking this drug. The U.S. Drug Enforcement Agency, or DEA, classifies substances according to the potential for abuse. The DEA categorizes methadone as a Schedule II narcotic, which means a methadone user may become physically and mentally dependent on this opioid.

Physical dependence means the person will suffer withdrawal symptoms if the level of methadone drops in his system. This decline in methadone levels could be because he skipped a dose, took an insufficient dose or took a medication that reduces methadone levels.

Methadone withdrawal symptoms are similar to other opioids and include:

  • Abdominal Cramps
  • Agitation
  • Anxiety
  • Blurred Vision
  • Insomnia
  • Restlessness
  • Sweating
  • Tremor
  • Vomiting

The most common side effects associated with this medication are not serious and disappear after taking this drug for a few days. Continue taking this medication but contact the prescribing physician if the non-serious side effects become intolerable or if they do not go away on their own. All formulas of methadone may cause the following non-serious side effects

  • Constipation
  • Dizziness
  • Drowsiness
  • Headache
  • Lightheadedness
  • Nausea
  • Vomiting

In addition to the above non-serious side effects, methadone diskets, methadone and methadone concentrate may cause dry mouth, increased sweating, itching or weakness. Methadone solution may be associated with dry mouth, weakness, increased sweating, itching, pain, redness or swelling at the injection site.

Serious Side Effects

Some side effects are serious, requiring immediate medical attention. Contact a doctor if you experience serious side effects. All methadone formulas may cause:

  • Excessive Drowsiness
  • Fainting
  • Seizures
  • Shortness of Breath

Contact your doctor immediately if you take methadone dispersible tablets and experience itching, pounding in the chest, slow heartbeat or sweating.

Methadone, methadone diskets, methadone concentrate and methadone solution might cause confusion, decreased sexual desire or ability, fast, slow, or irregular heartbeat, hallucinations, loss of appetite. Other serious side effects include menstrual changes, mental or mood changes, especially agitation, disorientation, exaggerated sense of well-being. Contact your doctor if you experience severe or persistent dizziness or lightheadedness, slow or shallow breathing, swelling of the arms, feet, or legs, trouble sleeping, trouble urinating, unusual bruising or bleeding while taking methadone.

In addition, a methadone solution consumer should contact a doctor if he experiences severe or persistent pain, redness, or swelling at the injection site.

Methadone as a Replacement Drug

When used as a treatment for opioid dependence, a patient receives methadone after his withdrawal symptoms have set in. It is important that the healthcare provider differentiate between symptoms of opioid withdrawal and methadone side effects.

Licensed specialists administer methadone according to the patient's size, overall health and sensitivity to methadone. Administering this first dose too quickly for the patient's individual sensitivity may result in adverse reactions.

By body system

Nervous System

Methadone acts on the central nervous system, or CNS, to cause pain relief and euphoria. CNS side effects associated with methadone include agitation, confusion, disorientation, dizziness, dysphoria, euphoria, seizures and sleepiness or insomnia. Methadone may rarely cause tardive dyskinesia or choreic movements, conditions that result in involuntary or repetitive body movements.


Methadone use is associated with cardiovascular side effects including low blood pressure, heart muscle disease, heart failure, inflammation in veins caused by blood clots and slow, fast or irregular heartbeat. Rarely, methadone use is associated with heart attacks.


Methadone may cause constriction in an important bile duct between the liver and gall bladder, worsening biliary colic. Methadone use is associated with gastrointestinal side effects such as constipation, abdominal pain, biliary tract spasm, dry mouth, inflammation of the tongue and appetite abnormalities.


Chronic methadone use is associated with excessive sweating.


Rarely, methadone causes hypersensitivity reactions that include itching, rash and swelling.


Some research suggests detoxification from methadone maintenance therapy may cause mood disorders in some patients. Other psychiatric side effects include psychological dependence on morphine.


Endocrine side effects have included stimulation of thyroxine binding globulin (TBG), resulting in increased TBG, total T3 and T4, but unaltered free T3 and T4. Hyperprolactinemia has also been associated with methadone therapy.


Genitourinary side effects have included stopped menstrual cycles, reduced sex drive or potency and urinary problems.


All opioids, including methadone, increase the risk for respiratory depression. Other respiratory side effects include pulmonary edema, which is abnormal fluid buildup in the lungs. Methadone use may cause hypercarbia, or high levels of carbon dioxide in the blood.


Methadone may cause side effects in the bloodstream including mild lymphocytosis, which is a condition characterized by high levels of a particular type of white blood cell. Methadone use is also associated with increased levels of protein, albumin and globulin in the bloodstream.


General side effects include weakness, swelling and headache.


Some methadone consumers experience Metabolic side effects including low potassium or magnesium levels. Others have reported weight gain.


Methadone may cause visual disturbances.