Agreed with Purges. You want to work your way up even if you feel the effects to be very mild at first, it's better than over-doing it and experiencing nausea. This can happen with Kratom preparations and its extracts, just as with any other substance whose mechanism functions primarily by stimulating the ([mu-] and [delta-]) opioid receptor sites.
Dosage of Kratom is a tricky subject, just take a look at the various graphs displayed on erowid:
http://www.erowid.org/plants/kratom/kratom_dose.shtml
A huge factor to be considered also is if any opiate tolerance/regular usage occurs in the Kratom-consumer, as this will greatly change the requirements for a much higher dosage.
I used to sell Kratom leaves, extracts and concentrates at a shop I managed. I had them brought in just to experiment and for those who were not opiate users (zero tolerance) always seemed to receive effects (some mild, some strong - nauseous strong*). Although again, dosage should be gradually increased and thoroughly felt out via relaxing, meditation or art forms each time in order to get a gauge. Nothing is worse than opiate-nausea in my opinion as its a constant uncomfortable dizzy-like-puke-drunk state except when/if one does vomit nothing changes, unlike the common occurrence of alcohol. Basically, opiate sickness has to be rode out and when I say that I mean rode out awake, "on the hip" (yes, like the opium-den days) until the psychoactive effects subside.
I've felt the effects of Kratom, along with my friends who love to experiment with psychedelics and various traditional/spiritual plant-medicines and we've loved it. When I sold Kratom I almost always received a positive review although what's most important is that the reviews were of great variety for the same opiate tolerance which seems to posit that each person(s) opioid receptor sites & the affinity of their strength is inherently, dramatically different between each person. So again I'll say: gradually increase your dose!
Try using either the same leaves, or extract concentrations while experimenting too as this will keep the variable stable. Even on erowid.org, Kratom (Mitragyna speciosa ** <-botanical name) there isn't accurate enough specifications between each different extract as companies are releasing new products with Mitragyna speciosa extracts differentiating everyday.
As a closing note I'll also say, opiate addicts have successfully used this as well to quell the sickness prompted from cessation of regular opiate-use. I've even heard stories of doctors recommending the herb and other heavy opiate addicts using it to kick their addiction fully. Always remember that the alkaloids primary affinity is within the opioid receptors and though has a somewhat tryptamine-like chemical structure and even feels tryptamine-like, Mitragyna speciosa consumed at psychoactive doses is an opiate. And so, regular use can cause the same effects as any regular opiate use. As is the same with using it to quit daily consumption. Though Kratom is much better than Methadone (for example), one would need to focus on decreasing their doses over time in order to rid themselves of the opiate-sickness bluez. A good friend used it instead of H. (diacetylmorphine) for a week and then could abruptly stop all consumption much more comfortably than cold turkey classically is without Kratom.
Blessed plants~~