Every investor likes to buy a stock and then seeing it generate a big gain. Every investor loves when the big gain comes quickly. And if a big gain can be made in only one week, it's even better.

Three stocks especially crushed it over the past week. Idera Pharmaceuticals (NASDAQ:IDRA), PetIQ (NASDAQ:PETQ), and Canopy Growth (NYSE:CGC) stocks soared 20% or more. Here's what lit a fire beneath these stocks and whether they're smart picks for investors to buy now. 

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1. Idera Pharmaceuticals

Shares of Idera Pharmaceuticals skyrocketed more than 40% this week. Investors can thank a Wall Street analyst for the huge jump.

JPMorgan Chase analyst Anupam Rama upgraded Idera stock to an "overweight" status with a price target of $15 per share. That reflects a 140% premium over the clinical-stage biotech's share price at the end of last week. Rama likes the prospects for Idera's lead candidate, tilsotolimod (also known as IMO-2125), which is being evaluated in a phase 3 study in combination with Bristol-Myers Squibb's Yervoy in treating metastatic melanoma.

This week's nice gain marks a much-needed reversal for Idera. Idera had hoped to merge with BioCryst Biopharmaceuticals, but the stock took a beating after BioCryst shareholders voted against the proposed deal.

2. PetIQ

PetIQ's share price soared 27% this week after the company reported its second-quarter results on Tuesday. As you might expect, those results were really good.

The provider of pet medication and veterinary services posted Q2 revenue of $171.1 million, up 96% over the prior-year period and easily topping the consensus analysts' estimate of $148.5 million. PetIQ's adjusted earnings of $10.6 million, or $0.66 per share, also blew away Wall Street's expectation of adjusted earnings per share of $0.38. 

To add icing to the cake, PetIQ boosted its full-year 2018 revenue guidance to $500 million, which reflects year-over-year growth of 88%. The company attributed its great results and positive guidance revision to the expansion of its pet preventative and wellness veterinarian products and services offerings and the opening of 17 wellness centers in the second quarter. 

3. Canopy Growth

Shares of Canopy Growth jumped 20% higher this week. Like PetIQ, Canopy announced positive financial results from its latest quarter. However, those results weren't the real reason for the Canadian marijuana grower's solid gain.

The big news was the announcement that major alcoholic beverage maker Constellation Brands (NYSE:STZ) was expanding its stake in Canopy with a $4 billion investment. As part of the deal, Canopy Growth will become Constellation's exclusive global cannabis partner.

Canopy now has a boatload of cash to fund its global expansion efforts. The company's CEO, Bruce Linton, listed several potential acquisition targets, including bottling operations in its home country. Canopy and Constellation also plan to launch cannabis-infused beverages in Canada when legally allowed to do so, which should be sometime next year. 

Are they buys?

Let's look at each stock in the order of their gains this week. First up is Idera. I like that the biotech is now focused on tilsotolimod, a promising TLR9 agonist. My view, though, is to wait for results from the late-stage study of the drug in combination with Yervoy. But aggressive investors might want to keep Idera on their radar screens.

PetIQ is one stock that I haven't had on my radar screen, but I wish I had. The company provides products and services for pets in retail channels that in the past were only available through veterinary clinics. It also operates a whopping 2,900 veterinary clinics of its own throughout the U.S. I like the market. I like PetIQ's retail strategy. And that makes me like this stock, even though it's a little pricey. 

Canopy Growth is a stock that I have closely followed for a while now. Don't look at this marijuana grower's valuation unless you like being scared. However, Canopy is set to be a major player in Canada's recreational marijuana market set to open in October 2018. The company already is a leader in global medical cannabis markets. The Constellation deal reinforces my view that Canopy is one of a very exclusive group of marijuana stocks that should be big winners over the long run -- although it could be a wild and bumpy ride along the way.