Syk is the street name of a forbidden circuit booster, presumably the Cybertronic equivalent of an illicit narcotic. Syk is forbidden even among the Decepticons.
Syk's effects (and negative side-effects) are unknown, but it may be similar to overclocking, where the increased strain on the boosted system carries a risk of burnout. Alternately (given its dealers), it may be chemical in nature.
Marvel UK comics continuity
While Soundwave searched for traitors by scanning the surface minds of his fellow Decepticons, he was surprised to learn that Mixmaster was still trading Syk on the base. Soundwave filed this fact for future blackmailing purposes.
Scrapper had Mixmaster pinned to a wall, telling a story about his brother, when Soundwave detected his thoughts. Presumably, this story was extremely boring, causing Mixmaster to wish fervently he was high right then.