This is the primary characteristic that makes methylnaltrexone behave differently than naltrexone.Furthermore, as methylnaltrexone cannot cross the blood–brain barrier, it does not reverse the pain-killing properties of opioid agonists or cause withdrawal symptoms, but since a small portion of analgesia comes from the peripheral opioid receptors, it can increase pain from inflammatory conditions such as arthritis. Side effects
The most common side effects for methylnaltrexone include:
Abdominal pain
Dizziness
Vomiting
Nausea
Diarrhea

History
In 1978, a dying friend and colleague presented the late University of Chicago pharmacologist Leon Goldberg with a clinical challenge. Struggling with the pain of prostatic cancer that had metastasized to his bones, the man was now declining the morphine he required for analgesia because of constipation. Research on opioids which would target only the sub-types of receptors associated with pain relief and not with side effects had seen little success outside of in-vitro models. Considering drugs such as loperamide, which acted on the opioid receptors in the gut without acting on the central nervous system, Goldberg proposed a targeted opioid receptor antagonist.Thousands of opioid-like molecules had been synthesized by pharmaceutical companies looking for the better analgesic - and many of those with no pain-relieving properties had been shelved. Screening these compounds led to the examination of putative antagonists which when modified had properties that suggested they might not readily cross the blood–brain barrier based on their size and charge. One of these compounds, N-methyl-naltrexone (MNTX), was amongst a group of compounds synthesized by Boehringer Ingelheim.
Treatment
Some babies recover on their own; however, some may require specialist intervention.Neonatal/pediatric neurosurgery is often required for avulsion fracture repair. Lesions may heal over time and function return. Physiotherapeutic care is often required to regain muscle usage. Although range of motion is recovered in many children under one year in age, individuals who have not yet healed after this point will rarely gain full function in their arm and may develop arthritis.The three most common treatments for Erbs palsy are nerve transfers (usually from the opposite arm or limb), subscapularis releases and latissimus dorsi tendon transfers.Nerve transfers are usually performed on babies under the age of 9 months since the fast development of younger babies increases the effectiveness of the procedure. They are not usually carried out on patients older than this because when the procedure is done on older infants, more harm than good is done, and it can result in nerve damage in the area from which the nerves were taken. Scarring can vary from faint scars along the lines of the neck to full "T" shapes across the whole shoulder depending on the training of the surgeon and the nature of the transplant.Subscapularis releases, however, are not time limited.