DMHA is a very strong psychoactive central nervous system based stimulant. Its first use was to treat nasal congestion, but for many years now it has gained traction and popularity by popping up and being used in different preworkout and fat burning products.
DMHA goes by many other names such as 2-Aminoisoheptane and Dimethylhexylamine. In addition to this, DMHA mainly comes in two forms, but brands tend to just label either as 2-Aminoisoheptane. You have:
SYNTHETIC VS NATURAL
The compound 2-amino-6-methylheptane is the synthetic form of DMHA, which you will mainly see in many preworkouts and fat burners. This version of DMHA tends to be the cheapest when it
comes to the manufacturing side of things.
The natural stimulant form of DMHA is called 2-amino-5-methylheptane, and even though it is the
natural version, according to some sources it's actually stronger than the synthetic as your body treats it differently. Furthermore, this version of DMHA can be found in many plant extracts such as Juglans Regia Extract (the most common), Kigelia Africana Extract, Persian Leaf Extract and Biophytum Root Extract. The cost to standardise a high percent extract of 2-amino-5-methylheptane is expensive, since naturally it is only found in low quantities.
DMHA provides a host of elevated functions including enhancing energy levels, enhancing brain function, decreasing appetite and improving bronchodilation (increased airflow). DMHA has also been shown to rapidly increase levels of dopamine and noradrenaline, two hormones within the body responsible for making you "feel good" or "feel euphoric", allowing you to perform much better than usual. DMHA has been on the market for a good while now and is more than likely to be banned due to its potency and the fact that America are in the process of banning it. As of the time of writing, Juglans Regia is legal in the EU.  
Does of DMHA vary from product to product, its usually anywhere between 75mg to 200mg. However, I have personally seen highs of 225mg right the way up to 350mg. Here at T-Nutrition, we stock a variety of pre-workouts consisting of forms of DMHA