When you have a significant amount of weight to drop or an imminent deadline (that class reunion really snuck up, huh?), trying to shed unwanted pounds quickly is pretty tempting. Here's what you need to know in order to do effectively and safely!

If you have more to lose, you’ll lose more—initially. It’s actually more useful to think of the weight you want to lose in terms of a percentage of your current weight, rather than a number of pounds. “For most men, a weight loss goal of 10 to 15 percent is a reasonable place to start,” says Nisha Basu, M.D., a primary care physician at Beth Israel Deaconess Medical Center and instructor at Harvard Medical School. So for a man who weighs 225, that would be 22 to 34 pounds. For one who weighs 350, he’s looking at 35 to 53 pounds—at least as a starting point. The bigger guy may also, therefore, drop more pounds in his first few weigh-ins. “Pace of weight loss is highly variable,” Basu says. “In general, though, for those with more weight to lose, initial weight loss can happen more rapidly.”

That first weigh-in may be dramatic. It’s not uncommon to see that scale needle make a satisfying downtick within the first week or two of changing your habits. “Generally the first week, most people can lose several pounds, which is mainly water weight,” says Basu. Why? When you put your body at a calorie deficit (i.e., eat fewer calories than you burn), your body immediately goes to its ready-energy stores of glycogen (basically, a form of sugar) to make up the difference. In the process, water is released. But once the glycogen is depleted and the body figures out it needs another way to find fuel, that’s when the actual weight loss begins.

Not all weight loss is equal. Aside from that tricky glycogen-fueled water weight, the body can burn both fat (yay!) and muscle (not-so-yay) as fuel. Not only that, burning fat for fuel isn’t nearly as easy at a cellular level as burning sugar... or protein (really not-so-yay). “Weight training and eating enough protein is key to not losing too much muscle mass,” Basu says. “Increasing your muscle mass can help to sustain the weight loss, too.” Which is why strength training may actually be more important than cardio in supporting a weight loss plan.

There’s a legit reason that pacing is key. You probably keep hearing about that whole 1-to-2 pound-per-week rule of thumb and think, “Psh, I can do better than that!” but hear us—and the Harvard doctor—out. “Almost every time a patient loses a large amount of weight in a short time, say 10 pounds in a week, the patient will gain it all back and more,” Basu says. “Further, several studies have shown that this yo-yo dieting is harmful to a person’s long-term health.” So basically, you may drop the lbs for the reunion, but you may be in a pickle to drop them again (and more) for the next one five years from now.

Super-intense plans aren’t so super. OK, but what if you really have to lose a lot of weight? See your doctor, and resist the urge to try what you may have seen on TV! “These extremely restrictive diets and very high intensity workout plans do not teach healthy eating behaviors or how to integrate exercise and activity into a person’s busy life,” says Basu. If you don’t learn how to make actual lifestyle changes, you’ll fall back into your old habits and…you know the rest. “In addition, the metabolism slows drastically in response to this type of weight loss, leading to many people regaining the weight even several years down the road,” she says. Which basically means an even harder path to weight loss in the future.

Everyone plateaus—and can persevere. So, unfortunately, you’re stuck losing weight at a slowish pace after all. An even more annoying reality check: Even that pace will inevitably stall out. When a plateau happens, it could be that you’ve let some old habits crop back up. Or you may simply need to make changes to what you’ve been doing, even though that exact plan was working so well mere weeks ago. “As a person begins to weigh less, he needs fewer calories to support the new lower body weight,” Basu says. But by simply tweaking portion size (smaller) or exercise (more), you’ll be back to losing in no time.