Buying from a reliable source who has consistent quality and purity substances is key although sometimes difficult to control. Ask how much MDMA is in the tablet, how strong the LSD is, how pure the 2C-B is or what kind of high the cannabis gives. There are several things to take into consideration when choosing a starting dose for a new substance. Many people are willing to jump right in with new substances, taking a full dose as calculated by well-meaning friends, in a set and setting where a bad reaction could cause significant disruption. If your intention is to be reasonably safe with your use of entheogens we strongly recommend against this.
People can and do have indiosyncratic responses to substances which cause others no problem at all.
Many people find threshold doses somewhat uncomfortable, as some of the physical and mental effects are present without being strong enough to fully immerse you in them. Effects to watch for include uncomfortably fast heartbeat, rash, hives or burning skin sensation, coughing or breathing discomfort, vomiting, or unconsciousness. If any of these effects do occur you'll need to determine, based on their seriousness, whether you need medical attention or can merely wait them out. Based on your reaction to this response test, you should be able to better judge what a normal dose of the substance would be for you. If you reacted fairly strongly to the threshold dose response test you'll know to start low.