From our friends at HowToGrowMarijuana.com:
Sativa vs. Indica Chemically
The high or stone sensation from smoking marijuana comes from molecules called cannabinoids.There are three main cannabinoids found in marijuana: Tetrahydrocannibinol (THC), Cannabidiol (CBD) and Cannabinol (CBN).
THC is the chemical in marijuana that gets you high, triggering a euphoric response when they bind to the many cannabinoid receptors throughout your body. In tests of pure THC, this molecule has been found to increase appetite, and suppress spasms, inflammatory responses and tremors.
CBD is not psychoactive like THC. It does offer a variety of medicinal effects including: suppression of nausea, suppression of convulsive, inflammatory and psychotic responses, sedative effects and protection from neuropathic diseases.
CBD moderates the psychoactive effects of THC and is said to help combat the paranoia that some users experience when smoking. CBD and THC interact to create different effects:
High THC:CBD ratio: the high is generally bright and energetic.
Low THC:CBD ratio: a sleepier, more narcotic stone sensation.
If both THC and CBD are high you get both mind and body effects creating a potent dreamy high.
CBN is produced as THC breaks down when cannabis ages and oxidizes. CBN is not psychoactive like THC, but it can increase feelings of drowsiness or a sluggish sensation of being drugged. Generally high CBN content is considered a negative result of improper marijuana storage.
Cannabis Indica has more CBD than Cannabis Sativa, giving it a lower THC:CBD ratio.
The higher CBD content is what gives Indica marijuana its body stone effects, as well as making it a valuable medicinal drug. Cannabis Sativa is more likely to be pure THC or have very little CBD, creating its euphoric mental high.