Michael Dickens’ Caffeine Tolerance Research

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Michael Dickens has read the research and performed two self-experiments on whether consuming caffeine builds up tolerance, and if yes, how quickly.

First literature review:

What if instead of taking caffeine every day, you only take it intermittently—say, once every 3 days? How often can most people take caffeine without developing a tolerance?

The scientific literature on this question is sparse. Here’s what I found:

1. Experiments on rats found that rats who took caffeine every other day did not develop a tolerance. There are no experiments on humans. There are no experiments that use other intermittent dosing frequencies (such as once every 3 days).

2. Internet forum users report that they can take caffeine on average once every 3 days without developing a tolerance. But there’s a lot of variation between individuals.

Second literature review:

If you take caffeine every day, does it stop working? If it keeps working, how much of its effect does it retain?

There are many studies on this question, but most of them have severe methodological limitations. I read all the good studies (on humans) I could find. Here’s my interpretation of the literature:

• Caffeine almost certainly loses some but not all of its effect when you take it every day.

• In expectation, caffeine retains ¹⁄₂ of its benefit, but this figure has a wide credence interval.

• The studies on cognitive benefits all have some methodological issues so they might not generalize.

• There are two studies on exercise benefits with strong methodology, but they have small sample sizes.

First experiment:

I conducted an experiment on myself to see if I would develop a tolerance to caffeine from taking it three days a week. The results suggest that I didn’t. Caffeine had just as big an effect at the end of my four-week trial as it did at the beginning.

This outcome is statistically significant (p = 0.016), but the data show a weird pattern: caffeine’s effectiveness went up over time instead of staying flat. I don’t know how to explain that, which makes me suspicious of the experiment’s findings.

Second experiment:

This time I tested if I could have caffeine 4 days a week without getting habituated.

Last time, when I took caffeine 3 days a week, I didn’t get habituated but the results were weird. This time, with the more frequent dose, I still didn’t get habituated, and the results were weird again! […] But it looks like I didn’t get habituated when taking caffeine 4 days a week—or, at least, not to a detectable degree. So I’m going to keep taking caffeine 4 days a week.

When I take caffeine 3 days in a row, do I habituate by the 3rd day?

The evidence suggests that I don’t, but the evidence is weak.