The consequences run on two separate tracks simultaneously, and both are serious in their own way.
The internal disciplinary track happens first and fastest. The inmate goes to the SHU while the investigation is underway. A Disciplinary Hearing Officer reviews the incident and determines the punishment, which typically includes time in disciplinary segregation, loss of good time credits, loss of privileges, and potentially a custody level increase that results in transfer to a higher security facility. That transfer is often deliberately placed far from home, which makes visitation difficult or impossible for the family and is used intentionally as an additional deterrent.
The criminal track is separate and depends on what was found and in what quantity. Possession of drugs inside a correctional facility is a criminal offense in every state and at the federal level. Depending on the substance and the amount, charges can range from a misdemeanor to a felony that adds real time to the existing sentence. A new conviction means a new sentence running consecutively to whatever is left on the original one.
The quantity matters considerably. A personal use amount is treated differently than a quantity that suggests distribution. The substance matters too. Fentanyl, methamphetamine, and heroin carry much harsher exposure than marijuana in most jurisdictions.
Cooperation is the single biggest variable in how this resolves. An inmate who works with investigators, identifies how the contraband came in and who facilitated it, often receives significantly more lenient treatment on both the disciplinary and criminal side. A few weeks in the SHU versus new felony charges and years added to a sentence is a meaningful difference, and cooperation is typically what determines which outcome applies.