Biotech giant Amgen (NASDAQ: AMGN) isn't starting the year on a strong note. The company's shares are down by 5% since Jan. 1. It has dealt with some issues lately, especially subpar financial results that have disappointed investors. Though Amgen remains one of the world's largest and most prominent drugmakers, its status alone isn't enough to make the stock a buy.

Let's look deeper into what's going on with the business to determine whether it is worth buying its shares today.

Can top-line growth bounce back?

Last year, Amgen's total revenue increased by 7% year over year to $28.2 billion. While that's a decent performance for a biotech giant, some of that growth was due to the company's acquisition of Horizon Therapeutics for about $28 billion. The acquisition closed in early October but had time to contribute $954 million to Amgen's sales. Excluding that figure, Amgen's total revenue rose by 3.5% year over year, which isn't particularly impressive.

Amgen has been dealing with competition for some of its medicines. The key will be to develop newer drugs. Amgen is working on that goal, including in the highly exciting weight loss area. One of its most promising pipeline candidates is AMG133, which is currently undergoing phase 2 studies. In an article published in Nature -- arguably the world's top scientific journal -- the authors presented results from a phase 1 study for AMG133.

Not only did it help patients lose weight, but they were also able to keep the weight off for up to 150 days following the last dose of the medicine. Amgen plans on releasing results from phase 2 studies later this year. The anti-obesity drug market is projected to soar in the coming years, with the condition rising to worldwide epidemic levels. So there is plenty of space here for Amgen to carve out a niche for itself.

The biotech is also working on many other programs. Newer approvals like cancer medicine Lumakras and asthma therapy Tezspire are still seeking label expansions. Overall, Amgen has 29 programs in phase 3 studies and a dozen in phase 2. It should earn important approvals in the next few years. The company will also help its new subsidiary, Horizon Therapeutics. Horizon's most important asset, Tepezza, for treating thyroid eye disease (TED), hasn't been on the market that long. It was first approved in the U.S. in early 2020.

Though it is the only treatment on the market for TED, there is a lot of work to be done. Amgen thinks about 100,000 patients in the U.S. could benefit from Tepezza, with many more abroad. The company has the sales force and funds to make things happen that Horizon Therapeutics, a much smaller drugmaker, didn't have by itself. That's why the increased adoption of Tepezza among TED patients could be another meaningful growth driver for Amgen.