By sixth year Sirius was doing heroin on a weekly basis, sneaking off to the Astronomy Tower or the lake or, if he was really desperate and wanted to be warm more than safe, the library. It was a secret both closely guarded and widely known, but James turned a deaf ear, until Sirius began over-using to a dangerous extent.
His third overdose raised suspicions in James that something other than alcohol poisoning was going on; he had witnessed the amount that Sirius could ingest without losing his head, let alone his blood alcohol level, and wondered that Sirius would even be able to obtain so much so regularly. The school had been taken in with this new Muggle trend, purebloods and Mudbloods alike, as the contemptuous Slytherins liked to pronounce. Rock n' roll had escaped Hogwarts, but sex and drugs abounded. Madame Pomfrey was forced, much to her shame, to acquire vast amounts of the Bubotuber pus so renowned a cure for the now-common STDs. Even with this, though, the way that Sirius was going was looking for trouble. Time and again he refused the help offered by his friends, unless they planned on stealing the potions necessary to keep him properly functioning.
Nevertheless, he was forced into the hospital wing almost as often as Remus. There were always plenty to keep him well supplied with flowers, candy, heroin: he could count on Lily to be the first one there when he was committed after yet another bout of the flu. His immune system was low, he was weak, and as far as she concerned that meant he would be that more eager to avoid withdrawal, and would pay that much more. She would have plenty left to finish Hogwarts with. She liked him well enough; it was just that she wanted to stay off the streets.