THE first time you meet Robin, it’s easy to be misled by her fragile appearance. She’s in her 40s and painfully thin, and she grips her aluminum walker with hands that have been twisted by rheumatoid arthritis. But she’s both tough and resourceful, and she doesn’t give up easily.

I met Robin and many others like her at a California medical marijuana clinic where I was doing research for a book. She was one of almost a dozen patients seeking recommendation letters that would let them buy marijuana at designated dispensaries.

I asked how marijuana helped her.

“I can’t live without it,” she told me.

She said it helped her sleep, and it relieved the constant pain in her joints. But Robin was most eager to talk about how medical marijuana put her in charge. She can decide whether to use it, when to use it and how much she needs. She doesn’t have to rely on a doctor.

“I’m in control,” she said.

That simple declaration might be a rallying cry for proponents of medical marijuana, which is legal in 23 states and the District of Columbia. By some estimates, at least one million people in the United States are registered medical marijuana users. There are likely to be many more who obtain it without registering, and still more who obtain marijuana illegally.