Thymalin

A mixture of low-molecular-weight polypeptides from calf thymus. It is understood to act as a regulator rather than a stimulant - nudging immune parameters back toward normal from either direction rather than simply pushing them up, which is the whole premise of the bioregulator idea.

Russian clinical literature, limited Western replication.

This is one of the better-evidenced things here, and unusually the evidence is human and long - Russian cohort follow-up over many years in older adults. It has not been replicated in Western trials, which is a fair caveat rather than a dismissal.

Thymalin has a different following from most compounds here - it is discussed by older users and in longevity circles rather than in gyms, and the reports reflect that. The dominant theme is infections: people say they stopped catching what went around, or that colds got noticeably shorter. It is run in short defined courses, typically about ten days, repeated a couple of times a year, and that rhythm is one of the most consistent conventions anywhere in this space - nobody runs it continuously and everybody says so. The second theme is that the benefit builds across courses rather than arriving inside one. Improvements in energy and sleep among older users come up often enough to be a real pattern. It is frequently discussed alongside Epitalon, from the same research tradition, and people often run the two in sequence.