dangers of drug use are damage to the astral body, loss of large amounts of jing, a weakening of the etheric, disorganization and damage to the chitta, spontaneous kundalini bursts and sudden incarnation of parts of the light body which have very high vibration. These dangers run along side the physical damage that can be caused to the brain, nervous system, internal organs and glands.
The most dangerous drugs in terms of damage to the astral body and chitta are hallucinogens like LSD or Magic Mushrooms. The states of consciousness induced by these drugs can be very high and can cause the light body to strongly incarnate into the astral body. As it does so the much higher vibration of the light body with its various structures of spinning light impacts into the astral and thereby increases its vibration very suddenly. If the astral body has weak spots or holes the light body will impact further and if it also has dense crystallizations it can crack and begin to shatter. As a result the individual may have an amazing spiritual awakening but when the drug wares off the light body will move back out and the astral will slow down again. If broken the astral body will never again be the way it was before the trip. Repair could take a very long time even with the right practices.
Other drugs such as ecstasy, other amphetamines and cocaine can cause the same type of problem but it's not as likely to be so sudden. The raise in vibration will not be as high with these drugs but when continually used the astral body can become very disorganized leading to a psychotic episode which may have the same results. These drugs when used repeatedly can cause holes and weak spots to become worse and crystallizations to become more brittle. This combination opens the ego structure up to possible psychosis.
Ecstasy is probably the worst drug of all when it comes to physical brain damage. As it releases huge amounts of serotonin from serotonin neurons it actually destroys the neurons. This destruction can be very quick and the implications are massive. Keep on taking ecstasy and pretty soon you will have very few serotonin circuits left and you are destined for depression. The lack of serotonin will prevent parts of the chitta from incarnating into the brain and so a disconnection from parts of your personality (located in the chitta) can occur. Repair of such brain damage is usually seen as impossible so if you do nothing it will not repair. I believe however that a regeneration of brain cells is possible through meditation and raising sexual energy