The first few times you popped a pre-workout supplement, you probably felt like Bruce Banner in traffic: Your body courses with energy, every muscle in your body feels supercharged, and suddenly you're pounding out reps in the weight room at six in the morning. 

But after a few weeks of the sweet, sweet pre-workout high, you're suddenly no longer hulking out of your gym shorts. In fact, the same pre-workout dose doesn't deliver much more than a baby buzz. 

What gives?

Part of it is simply psychological: Humans are remarkably adept at sensing and adapting to changes in our environments, explains nutritional biochemist Shawn Talbott, Ph.D., L.D.N., author of A Guide to Understanding Dietary Supplements. When you're running on empty and start taking a pre-workout supplement, the boost in physical energy, mental focus, endurance, and blood flow are all brand new feelings. “We sense this change very dramatically the first few times it happens, but then we become accustomed to the feeling, so subsequent uses of the very same product at the very same dose will feel 'less' in terms of magnitude of effect,” he explains.

But there's also a physiological and biochemical change: When you take the same pre-workout formula nearly every day, your cellular receptors become saturated and nerve endings desensitized, both of which reduce the magnitude of those effects, Talbott explains.

It's inevitable—your body and brain will become accustomed to initial doses of caffeine, arginine, beta-alanine, and niacin pretty quickly (within a few weeks), he says. But it's probably the caffeine conditioning that's hitting you hardest, since the invaluable energy is the most common perk of pre-workout supps.

Most guys rely on a simple solution: take more. But piling on the powder won't fix the dulling effects. Once those cell receptors are saturated and nerve endings desensitized, the boosts just aren't as powerful. “Going from zero to 100mg of caffeine will deliver a very noticeable lift in energy for most people, but then up to 150mg delivers only a fraction more—and up to 200mg only a fraction more of that, eventually getting to the point, above 200mg, where tension and anxiety start to counteract the energy benefits of caffeine,” Talbott explains. 

A better option: Delay that dulling sensation from the start by choosing a pre-workout formula that has fewer than 100mg of caffeine per serving, as well as ingredients that extend the duration of caffeine, like arginine, a blood flow enhancer, or focus-boosting ingredients, like tyrosine and BCAAs.

So, how do you get your boost back? “If your favorite pre-workout supplement isn't performing for you the way that it once did, go off it for two or three days,” Talbott recommends. That's probably good advice anyway, since taking a pre-workout supplement regularly has only been confirmed safe for up to 28 days. Taking a pre-workout for longer isn't unsafe—it's just that researchers have yet to study longer periods of use in a clinical environment.

Another option, “Try another formula that works through different mechanisms,” Talbott says. Most pre-workout supps blend caffeine, beta-alanine, and arginine, so consider a formula that delivers energy from different ingredients. Talbott formulated a stimulating tea that combines ephedra—the legal kind—and New Zealand pine bark to deliver more of a mental energy for your workout than the total-body kind you feel with the typical PWS blend. 

Also consider a formula with the naturally energizing yerba mate, which one British study found can also help increase fat oxidation without compromising power during an aerobic workout; Brazilian research says yerba mate can help you recover from a strength session faster, too.