Thanatos

Named after a deity of death of one of the Old Faiths, Thanatos is the famous drug used extensively by the Core Units of the Legion to improve combat effectiveness of its troops. It is a synthetic drug similar to amphetamines that was developed by the Legion's chemists. Obtaining it is possible for outsiders if they steal or loot it. Additionally, in some places bootleg imitations of the substance are circulated.

Immediate Effects & Side-Effects

In very low dosages, it increases drive and focus, alertness, and can help with staying awake. The effect is very short-lived, however.
With increasing doses, it lasts longer and the effects become more intense. At the "recommended" dose, the drug induces a state of extreme focus and wakefulness, suppressing exhaustion and even pain. Reaction time is increased as well. A lot of users experience euphoria, though overly aggressive behaviors can also be observed on occasion.
An overdose causes the effects to loop around into the adverse. The person quickly becomes extremely irritable and confused, may feel faint. Loss of muscle control, vomiting, a severely increased heart rate, becoming completely unresponsive, or passing out occur. In most cases, an overdose is lethal.
Common side-effects for all dosages include weight loss (due to a loss of appetite or an inability to keep food down), changes in sex drive and varying degrees of insomnia. Addiction, both physical and psychological, sets in very quickly.
Once the effects of the drug start to wear off, an intense drowsiness is experienced. It can be strong enough to make the user unable to stop themselves from falling asleep.

Administration

For ease of use - and because it has been found to be the most fool-proof way of preventing soldiers from messing up the proper dosage - Thanatos is usually handed out in the form of pills. There is only one version of the pill available, but it features a notch that allows splitting it to account for different body mass.

Item type

Drug / Narcotic / Medicine

Long-Term Effects

Despite many of its users dying in combat before any noticeable long-term effects can set in, these effects are nevertheless pretty well-researched.
Those who don't get killed show clear signs of deterioration in the span of only a few years of regular use. Severe organ damage (heart, liver, kidneys, etc.) to the point of failure occurs in all cases.
The neurological damage of prolonged use is also easily apparent: Long-time users develop all kinds of psychological and behavioral issues, including paranoia, psychosis, compulsions, obsessions, flat affect or inability to regulate emotions, and the like.
When the recommended dose is exceeded, these symptoms may develop in a much shorter time. Attempts to postpone the onset by mandating "detox breaks" in which the user is supposed to recover at least partially have proven counter-productive, as the withdrawal symptoms are generally too severe to manage.