I beg to differ. Opium or rather alkaloids its contains is "benign" because it merely substitutes endorphins that are naturally produced by the body. The problem is with impurities and OD - most of the street stuff is contaminated with foreign substances that may produce unpleasant side effects after IV injection. HIV infection would be in the same category.
Another problem is OD - after a while you need megadoses to get the same high and that can easily lead to overdoses.
Aside overdosing, opium & derivatives (morphine) per se do not produce any damage to the body but they produce behavioral changes that may be detrimental. One of them is suppression of hunger - a person can go without food for days without feeling hungry - which may result in severe malnutrition. Other behavioral changes are reduced sex drive and motivation.
Opium was regularly prescribed and used in the US in the 19th century, without much side effects. It got on the verboten list only after Chinese workers entered this country to build the US railroads. They smoked opium - peddled to them by Her Majesty government - to kill the pains of hard work and loneliness (as they spouses were not permitted to come with them).
The ban was a part of the anti-immigrant anti-foreign hysteria that periodically engulf the noble folk of this land. Weed was banned for similar reasons, because Mexicans smoke it. The prohibition was a result of another anti-immigrant hysteria directed at southern and eastern European immigrants. Surveiller et Punir. Making sure that all little pleasures are methodically taken away seems to be an integral part of the American Puritanism's modus operandi.
Wojtek