The effects of kratom have been reported to feel like codeine or other mild opiates. This could be because the active alkaloids (7-hydroxymitragynine and mitragynine) act on opiate receptors. You feel both stimulation and sedation at the same time. You can feel relaxed and calm yet you can work harder and longer. The most enjoyable effect is the euphoria which is similier to codeine or hydrocodone. Some possible negative effects are reported to be bitter taste, Dizziness, nausea and/or vomiting at higher doses, and a feeling of heat in your body. Onset takes between 10-30 minutes and the effects last between 2-5 hours with about 2-5 hours of mild after effects.
People who use kratom for several days in a row can build a tolerance to the effects. Tolerance means it will take more kratom to reach the same amount of effects. I would advise using kratom in moderation and always be responsible while using kratom. Even though kratom is a all natural leaf, it is still a drug and should be respected.