You say they are called dangerous, well as I understand it ludes / quinazolinones are one of the first generations of downers after simply smacking someone over the head with a blunt object or perhaps chloral hydrate... after that came barbiturates
barbiturates came before quinazolinones for use in medical practice as sedatives, hypnotics, and anti-anxiety agents. methaqualone was discovered in the early 20th century in India while looking for a malaria treatment. barbiturates were discovered in the late 1800s and were being commonly used by the time methaqualone was just discovered, and it wasn't untill the mid 20th century methaqualone appeared for use as a sedative/hypnotic.
methaqualone was actually put on the market because problems with the safety of barbiturates were quickly on the rise, so methaqualone was introduced as a safer non-addictive drug just as effective as barbiturates.
as time passed methqualone was being commonly perscribed as a barbiturate alternative and soon users discovered methaqualone's recreational effects. after a while quaaludes became one of the most abused drugs and peaked in popularity in the 1970s especially in disco clubs(why they were nicknamed "disco biscuits"). quaaludes also were known for having pro-sexual effects such as increased arousal and sexual desire.
quaaludes were pretty much the first "ecstacy" type drug as it was a popular club and party drug.
by the time the controlled substance act was developed methqualone quickly was placed in schedule 2 after continued reports of abuse, dependence, and overdose as well as it's illicit sale on the street and popularity as a recreational drug. soon methqualone became considered to cause more problems than good and it was placed in schedule 1.
now authentic quaaludes in the U.S. are more uncommon, although they do exist due to diversion of legitamite methaqualone from countries that still use it(South Africa, Canada, among others). fake quaaludes usually contain benzodiazepines, usually diazepam at doses anywhere from 30-100mg.
funny how a drug introduced as a safer alternative to one drug turned out to have even more abuse potential.