CDC Report. suggests the problem is with overdoses and when mixed with alcohol or other drugs.
more on Mitragyna at wikipedia.
like any medicine, it can be used (i.e. to treat symptoms or to improve one's ability to live a normal life) or abused (to get high).
It is commonly used in Thailand and Malaysia, where psychiatrists note the problems in chronic users:
Results: The majority of the subjects were male, age 18–35 (39.1%), and agricultural field laborers (65.5%). Kratom was commonly used by chewing (49.6%), together with drinking coffee (69.0%).
The common clinical effects reported from the use of 1–20 leaves included psychoactivity (69%), decreased muscle pain (63%), and sunburn sensitivity (59%).
Stimulant effects were more prevalent at low doses, while opiate-like effects were more common at higher levels of use.
withdrawal symptoms were reported, including muscle aches (68%), insomnia (60%), and irritable mood (59%).
However, taking more than 20 leaves per day for 3 to 5 years resulted in reports of hallucination (31%) and paranoid symptoms (17.2%).
Continuing abusers for more than 5 years often had small-sized feces similar to goats', and some reported decreased euphoria and impaired cognitive abilities.