Joni Mitchell Mercury Poisoned

Quoted from the above article:

If Morgellons is indeed related to a psychiatric condition, drug use may be a contributing factor, the researchers said. In the hair samples in 50 percent of patients in the study, the researchers found evidence of the use of drugs such as marijuana, amphetamines and benzodiazepines (which are used to treat people with anxiety and insomnia).

Mercury poisoning messes with the ability to produce enzymes for normal body chemistry, so it's not surprising we would see a higher incidence of drug use as the sufferer attempts to stabilize their own body chemistry.
Common symptoms of mercury poisoning include peripheral neuropathy (presenting as paresthesia or itching, burning or pain)