Not an older member, but I'm an amateur historian...
Methaqualone (quaalude,mandrax) Quinazolinone sedative. Similar in action to barbiturates, but technically of a different class of drugs. Overdoses very possible. Coordination is said to be severely impaired and sensations of pins/needles/numbness are experienced. Although, I have no empirical knowledge of this stuff. They still use this stuff in South Africa, where it's made illicitly and sold in tablets resembling "ecstasy". In fact, they even smoke it; it's called "white pipe" or something like that.
Secobarbital/Pentobarbital/Amobarbital etc. (Seconal, Nembutal,Amytal,Tuinal) Powerful barbiturate sedatives of relatively short action. Stronger/more dangerous cousins of benzodiazepines. Overdose being a very real possibility. Barbs today include phenobarbitol and butalbital - other barbs are technically still available, but in practice are almost never prescribed - both of which are long-acting and nothing to write home about. Even these are beginning to be phased out it seems. The advent of chlordiazepoxide (Librium) and especially of diazepam (Valium) was the death sentence for barbiturates ;sedative hypnotics with a more solid safety profile; overdose unlikely unless combined with other CNS depressants.
Heroin (morphine diacetate) We all know this one. W. Burroughs stated in junkie that the heroin caps he sold were of about 16% purity. This was considered unusually strong stuff according to him and the police in the novel. Also, he lived in NYC, which has historically been the heroin capitol of this country. A conservative guess would say even garbage dope is twice as potent in the present day and carries a much cheaper price tag. Burroughs sold his caps for $2.00 (~26.00 present day) when a bag today costs roughly XXX. Not to mention, hitting up doctors for prescriptions of morphine/opioids was much more easy/common.
Cannabis. A couple older guys I've worked with claim that weed was better back in their day. I don't understand how they can comment if they don't even smoke? The quality seems to have steadily improved over the years. When I used to cook, an older line cook told me when he would buy pot he'd buy a kilogram and he said this was not uncommon. So you can only assume the stuff was of poor/mediocre quality.
Amphetamines (Benzedrine, Black Beauties other names) Benzedrine inhalers were sold over the counter and contained a whopping 325mgof racemic amphetamine (50/50 d,l). I've read that consuming the contents of an entire inhaler was not uncommon. It should be noted that benzedrine was also available in the form of tablets. I believe the OTC sale of benzedrine and like amphetamine preparations ended sometime before 1970, but was being phased out years before then.
Paregoric (Camphorated Tincure of Opium, Odorized tincture of Opium) This weaker, more dilute cousin of Laudanum (Tincture of Opium, typically 10% raw opium by volume in ethanol) could be bought at certain drug-stores on a "case by case" basis. Those addicted to stronger opiates/oids could use paregoric to ease their symptoms of withdrawal. I understand codeine preparations are still sold OTC in some areas, but I'd say the relatively low strength renders the stuff useless to those with a habit.
If you go far enough back in history, say, before the Harrison Narcotics Act of 1914, all drugs/psychoactive substances were essentially legal and were bought and sold just like any other commodity. I've always said, I would like to have lived around the turn of the century. It would be an opiate run for the history books I'm sure...
