Heroin
  Heroin is a euphoric depressant. It depresses the functioning of the Central Nervous System, slowing down bodily functioning and reducing physical and psychological pain.
At least before tolerance levels are greatly raised, users experience a feeling of euphoria (a rush) shortly after taking it especially if injected.
Smaller doses produce feelings of warmth, well being and a slight detachment. Larger doses cause heightened relaxation and sleepiness, sometimes referred to as 'gouching' or being 'on the nod'.
All opiates also cause repress the cough reflex (which is why codeine is often found in cough mixture) and cause varying degrees of constipation).