Marine Activation Drug (MAD)

Marine Activation Drug (MAD) is administered to Starship Marines as they go into combat by an injection unit inside the armoured suit.
MAD consists of pseudo-natural substances, similar to endorphins, that suppress the natural fear reactions. They also suppress pain, discomfort and boredom, and enhance alertness and reflexes. Unlike the highly non-organic chemical drugs of earlier centuries, these, being 'natural' substances are non-addictive and have few side effects (see below).

This is coupled with personality modifying psychological preparation like advanced neuro-linguistic programming and deep hypnosis to uncouple the 'natural' instincts to panic and run away and leave the marines free to act intelligently. Their enhanced rationality makes them more obedient too.

Side Effects

Side effects of the Marine Activation Drug include: controlled emotional state, zero sex drive, some sociopathic tendencies (due to a loss of empathy - they don't care if their mates die - or anyone else for that matter).
When coming off the drug at the end of operations there are some residual grief emotions to cope with, but the marine corps has rituals and depth counselling to assist with this. And some recreational drugs too. The equivalent of getting blind drunk after a battle has a long and honourable history.

The MAD wears off fully after 24 hours, and the long term permanent side effects are minimal (and these may not be down to the drug - like difficulty in forming long term relationships with non-marines).
There is, for those with difficulties, the commercially available Sexual Enjoyment Neurological Enhancement (SANE) drug, which is, in many ways, the opposite of MAD.

WHY DON'T NON-MARINES USE MAD?

The high morale provided by MAD is the main edge marines have over, for instance, pirate combat crew. Not only is MAD is very difficult to make (and therefore very expensive), its use by those not pre-prepared by a combination of psychological profiling at the selection stage, hypno-therapy and NLP techniques during training is highly dangerous. Use by anyone not highly prepared will induce bipolar behaviour or serious psychotic episodes. Some of the effects of un-prepared use of MAD are permanent. As it says on the packet - improper use of MAD will make you MAD.