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Ever since the Pharmaceutics Law passed, everyone in the country has unlimited access to a dizzying array of mind and mood-altering drugs. A shot of Slumberine helps you sleep, and any emotion you can imagine is just a pump away. Even vacations can be replaced with shots: take one dose, and your mind is bathed in a relaxing hot springs experience. Amidst all this drug-addled merriment is Kabu, a world-weary twenty-something man, with nothing to do but chase his next big high. But one day, after running out of P, his drug of choice, he experiments with a new super-drug, Ultra Heaven, which completely re-writes his sense of reality and changes his life forever...
Susamu Nakoshi, a man disillusioned with life, meets Manabu Ito, a young medical student bored with life. To relieve his boredom Ito offers Nakoshi 700,000 yen to undergo a trepanning to see if it will unlock his sixth sense, and Nakoshi eventually agrees. However, much to his dismay and chagrin it actually succeeds, and now Nakoshi must contend with seeing homunculi, the metaphysical and symbolic representations of a person's psychological disorder. He now helps people, usually against his own will and often theirs as well, with resolving their psychological problems.
These two seinen manga are about bored young men, disillusioned with the world. And while one focuses on psychopharmaceuticals, and the other on psychoanalysis, both deal with what's going on in people's heads, and are fairly dark. Besides that, their plots aren't terribly similar, but I think they might appeal to the same audience.
These manga share a common style because how the events are mainly described by the images, since both main characters experience an alteration on the perception of reality (by drugs in Ultra Heaven, by a surgical procedure in Homunculus).
Though not particularly similar in terms of plot (Ressentiment is a depressing story about otaku, while Ultra Heaven is a slightly-less depressing story about drug addicts), both are seinen soft science-fiction titles that explore a near-future world where one particular facet of life has been highly developed (video games/drugs).
Neither have terribly likeable protagonists, nor are light fluffy reads. Additionally, they both have a similar split between events that are actually happening, and stuff that's going on either in a video game or in the protag's mind. (The third volume of Ultra Heaven in particular is a lot like the rpg-world of Ressentiment)
The main character in both of these manga spends a significant portion of the story in a drug-induced stupor. If you enjoyed the trippy art, and not quite knowing what's actually happening and what's just an effect of the hallucinogens, definitely check the other out.