What goes up must come down. Those were the words Sir Isaac Newton used to describe the forces of gravity, but more often than not, those words also can be used to describe highflying momentum stocks.

When a momentum stock gets hot, valuation doesn’t matter — at all. Price becomes completely divorced from value. If you’re an aggressive trader, that’s OK. Your holding period is likely only weeks…or maybe even days.

But herein lies the key. You have to know when to sell. Because if you hang on too long, all of those outsized gains can disappear in a hurry, and you can be left nursing some nasty losses.

Consider the case of Chinese stocks. Between March and June, the Deutsche X-trackers Harvest CSI 300 China A-Shares ETF (ASHR) jumped by nearly 55%. By late August, it has given all of those gains back … and then some.

Today, we’re going to look at five stocks that have, at least until recently, been racking up significant gains. If you own any of them, I recommend you sell them, or at the very least, tighten your stop losses.

Sure, they could go higher. But at current prices, there’s a lot more downside than upside.

Netflix, Inc. (NFLX)

I’ll start with streaming content pioneer Netflix (NFLX). I love Netflix. I’ve been a customer for years, and I have nothing but respect for CEO and founder Reed Hastings. Hastings is a true visionary who was willing to effectively destroy his original business — renting DVDs by mail — to create something bigger, better and far more revolutionary: streaming movies and TV shows over the Internet.

Hasting and his team have fundamentally changed the way we consume content, and they’ve created fantastic original content of their own. I now spend far more hours watching Netflix than I do watching traditional TV. And they’ve managed to do all of this while charging their customers a negligible $7 per month.

But no matter how much I love the company, I just can’t love NFLX stock at today’s prices. Netflix trades for 7.5 times sales and 425 times next year’s expected earnings. Not surprising, given that the stock has more than doubled this year.