There are a lot of healthcare companies that are investing in GLP-1 weight loss drugs. Today, the leaders in that space are Eli Lilly and Novo Nordisk. But it could get much more crowded in the future. And it's little wonder why, given analysts project that the obesity drug market could be worth $200 billion by 2031. It's a mammoth growth opportunity that can jump-start a company's sales and profits.

Before drugs are approved, investors can gain insights into just how promising a GLP-1 drug is from the results of clinical trials. One drug I've been keeping a close eye on, MariTide, comes from Amgen (AMGN 4.07%). Unlike the current products on the market which require weekly injections, Amgen's MariTide is taken monthly, and it has the potential to be a much more attractive option for patients.

But its recent results were a concern. And they could end up giving investors more of a reason to invest in Eli Lilly and Novo Nordisk instead.

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MariTide's big problem: side effects

Prior to these recent clinical trial results, I was optimistic that MariTide could make Amgen an underrated stock to own. Patients having just a monthly injection which might yield similar weight losses to that of drugs currently on the market would seem like it should be able to win over patients and doctors.

Phase 2 trial data has shown that MariTide can help people lose around 20% of their body weight after being on it for about a year. Recently, the company released the full data from that trial, which uncovered a concern: At the highest dosage, 27% of participants stopped taking it due to gastrointestinal issues. But the company says that when the dosage was slowly increased, the discontinuation rate was less than 8%.

The company will ensure patients in the Phase 3 trial, which is currently ongoing, will increase their doses in an effort to minimize side effects. How high that discontinuation rate ends up being in a larger trial could be critical for the stock.

Why this should raise concerns for investors

When it comes to GLP-1 treatments, these are medications that patients need to be on forever. The biggest downside with these treatments is that someone who stops taking them can end up gaining back the weight that they lost -- perhaps even more.

If someone can't tolerate the treatment then there will be the inevitable temptation to select a different option, one where the side effects may not be as troublesome, even if it means more frequent injections or slightly lower weight loss. While a lot of the focus is on sheer weight gain percentage, I think the biggest winners in the GLP-1 race will be the companies whose treatments are best tolerated. An extra 5% weight loss may not be worth it if the trade-off means more severe gastrointestinal issues or other side effects.

Eli Lilly and Novo Nordisk, which already have approved GLP-1 weight loss drugs that are raking in billions of revenues, have set the standard thus far. MariTide isn't approved, but even if it is, if it doesn't shake concerns related to side effects, then there may not be strong demand for it.

Has Amgen become a bad buy on this news?

Amgen was looking like a potentially underrated growth stock to buy given the potential for MariTide. It still has a good business and MariTide may recover, but based on this recent data, I'd take a wait-and-see approach with the stock for the time being. The GLP-1 opportunities are what made it a compelling investment, but now, amid the question marks around MariTide, the bullish case just isn't that strong anymore.

If you're looking for a top healthcare stock to buy that has much more upside in GLP-1, then going with Eli Lilly or Novo Nordisk can make much more sense at this stage. They have solid, proven products that are already approved in the space, and their share prices could end up rallying on this news.