Brief Answer:
Probably depends on who was writing the prep instructions.

Detailed Answer:
Hello,

Different surgical departments write their own prep instructions and some may be more concerned about substances used before anesthesia than others because the nurse anesthetist or whomever writes that department's instructions is more concerned, not that it is more pertinent for that procedure over another.

Common anesthetics used in the anesthesia for a colonoscopy are a benzodiazepine such as midazolam, a sleep inducing medication such as propofol, and a narcotic pain reliever such as meperidine. Different clinics/hospitals use different combinations of these types of medicines.

Marijuana has the potential to increase the amount of these medications in the blood or to increase the length of time before they are broken down. They have synergistic effects. The effect may not be severe, but may make things more unpredictable. So the anesthesiologists or nurse anesthetists for the department that does the colonoscopies may not want to deal with this risk.

Any medication that competes with anesthetic medications for liver enzymes and sites where it they are broken down pose a risk for synergy or antagonism with the anesthetics, and that's why you are asked about what medications you are on before any procedure. Given this knowledge, the anesthesiologist can make little adjustments in what and how much of the medications they give you.

I hope this explanation helps. Please let me know if I can provide further information or clarification.

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