What drug was Sherlock on? (Obvious warning for frank talk of drug use)
Many people have long assumed Sherlock was addicted to cocaine due to Doyle canon. However, I think we got clues in His Last Vow that his drug of choice is something else: Heroin.
First, when John is about to go into the drug den, he refers to the people inside as “smackheads,” which is slang for heroin users. It could be a wild guess, but consider that John is a doctor and he does know the person he’s looking for. It’s established that both Mary and John know Isaac and his history with drugs (Isaac recognizes them both and it was implied that his mother had spoken to the Watsons about the issue before). John could have said cokeheads, he could have said the much more general “junkies,” but he specifically referred to smack. I think he did so because he knew Isaac was on heroin specifically, and - as drug users typically segregate themselves based on their preferences - I think John knew perfectly well what was going on in that house.
Second, when Magnussen scans Sherlock, one of his “pressure points” is Opium:
Now that’s a reference to The Man with the Twisted Lip, where Sherlock is caught by Watson in the opium den. However, this is 2014 and a modern updating - we don’t really have “opium dens” anymore.
Heroin (diacetylmorphine or morphine diacetate (INN)), also known as diamorphine (BAN), and colloquially as H, smack, horse, brown, black, tar, and other names, is an opioid analgesic synthesized by C.R. Alder Wright in 1874 by adding two acetyl groups to the molecule morphine, found in the opium poppy. It is the 3,6-diacetyl ester of morphine. Heroin itself is an active drug, but it is also converted into morphine in the body.
When used in medicine, it is typically used to treat severe pain, such as that resulting from a heart attack or a severe injury. The name “heroin” is usually only used when being discussed in its illegal form. When it is used in a medical environment, it is referred to as diamorphine.
If Sherlock had been addicted to cocaine, his weakness would’ve been listed as cocaine. But instead opium is listed, which is the alkaloid used to produce heroin. Cocaine is not an opioid at all.
Third, much is made in the episode of Sherlock’s (medicinal) use of morphine (an opiate) after the shooting, including Janine’s dig that the hospital must be nice for Sherlock since they hook him up to the drugs themselves. And what do we say about coincidences?
Fourth, there are the effects. Pushing any personal experiences aside for the moment, the behavior of the people in that house was much more in line with smack than coke. Those people were sprawled out, dazed, and no doubt feeling no pain. Sherlock was curled up on a mattress, either zoned out or sleeping. Heroin can make you feel relaxed, like you’re floating on a cloud free from pain, calm, warm and comfortable. That’s often why it’s so popular among the homeless and prostitutes. OTOH, cocaine is a stimulant. It makes you hyper-alert, energetic, talkative, more sensitive to sensations. You don’t pass out on a dirty mattress on coke, because you can’t sleep.
We also saw that the people in the house were using needles and had track marks on their arms. Intravenous use of heroin is much more common than IV use of cocaine, both because it’s harder to calibrate the correct dosage of coke intravenously, and because even drug addicts know that using needles comes at a greater health risk. They’re not going to use an inconvenient method unless they have to. Coming down from opiates can also induce feelings of irritability and aggression, which would explain Sherlock’s physical attack on Mycroft.