The most common supplement form is creatine monohydrate. This is the form that has been used in the majority of research on the topic.
This means that most of creatine's beneficial effects, such as improved upper and lower body exercise performance, have been observed almost exclusively when creatine monohydrate was used.
This form is made up of a creatine molecule and a water molecule, though it can be processed in a few ways. Sometimes, the water molecule is removed, resulting in creatine anhydrous.
The removal of water increases the amount of creatine in each dose. Creatine anhydrous is 100% creatine by weight, whereas the monohydrate form is about 90% creatine by weight.
Other times, the creatine is micronized, or mechanically processed to improve water solubility. In theory, better water solubility could improve your body's ability to absorb it.
Despite these minor differences in processing, each of these forms is probably equally effective when equal doses are given.
In addition to increasing strength, creatine monohydrate can increase water content in muscle cells. This may lead to beneficial effects on muscle growth by sending signals related to cell swelling.
Fortunately, a large amount of research indicates that creatine is safe to consume, and no serious side effects have been reported with its use.
When minor side effects do occur, they typically involve an upset stomach or cramping. These side effects may be relieved by consuming several smaller doses, rather than one larger dose.
Because it's safe, effective and affordable, creatine monohydrate has long been the gold standard for this supplement.