Inhalation of any kind of smoke is unhealthy.
Smoke is composed of fine particles of a solid dispersed in a gas.
The lung acts as a filter and retains the solid particles for varying periods of time. So, for each joint smoked, more of the harmful ingredients found in both marijuana and tobacco are absorbed into the body of the user. Many other compounds have been identified in marijuana that are not found in tobacco smoke. While some of these may eventually be used as medicines the effects of most on the human body are unknown. There is no quality control in the preparation of marijuana for sale and consumption.