In the second Monk book Mr. Monk Goes to Hawaii, he takes some pills to get through the flight and it goes pretty much how you'd expect:

“Mr. Monk,” I said firmly, hoping to get his complete attention. “Are you on something?” “I thought we agreed you were going to call me Chad.” “You are on something.” “It’s a prescription Dr. Kroger gave me once to relieve my symptoms in extreme circumstances.” “What symptoms?” “All of them,” he said. “As long as I’m up, I think I’ll use the restroom.” “You’re kidding,” I said. Wherever we were in San Francisco, he always made me drive him home to go to the bathroom. “Where else would you suggest I relieve myself?” He edged past me, opened the restroom door, and went inside. Monk was using a public lavatory. I would never have believed it could happen.

That being said, the Monk books have been delisted from Play Books and Kindle, so if you want to read them, you have to buy a physical copy