Just a few weeks ago I was celebrating the success of my investment in GSK (LSE: GSK) shares. Suddenly, they’re not sitting quite so pretty. What happened?

I bought the FTSE 100 pharmaceutical giant in March 2024 precisely because the shares had struggled for years. Former CEO Emma Walmsley froze the dividend and funnelled cash into research and development to rebuild GSK’s weakening drugs pipeline.

Other issues hurt sentiment too. Investors worried about patent expiries, legal battles over its Zantac heartburn treatment, sluggish vaccine sales after the pandemic, and fears that the company lagged rivals in breakthrough obesity drugs.

Why did this FTSE 100 stock finally take off?

It was a thankless task for Walmsley, and the shares never really took off under her leadership. Still, I bought because the valuation looked tempting, with a price-to-earnings ratio of around eight. I was disappointed by the lower yield. The days of 5%-plus dividend income had long gone. I locked into a yield of just over 3%, hoping shareholder payouts would improve as new treatments boosted revenues.

The shares drifted lower for months before bouncing after Walmsley stepped down in December 2025. Full-year results published on 5 February this year were encouraging. Sales rose 7% to £32.7bn while core operating profit climbed 11% to £9.7bn. Free cash flow surged 41% to £4bn.

Net debt crept up to £14.5bn, higher than I expected. But the first results published under new CEO Luke Miels suggested that GSK might finally regain its momentum. Management kept its long-term 2031 sales target above £40bn intact too.

Yet the last three months have been bumpy, and the shares have fallen 12%. That would shrink a £13,000 investment to about £11,440, a paper loss of £1,560. While disappointing, it’s not a huge issue. Shares move up and down all of the time. Over 12 months, GSK share price is still up 27%.

I plan to hold my my shares for years, hopefully decades. Ageing populations and rising healthcare demand should create huge opportunities over time. GSK also has exposure to the massive US market, where it generates roughly half of all its revenues.

Should investors consider buying GSK today?

First-quarter results on 30 April showed progress. Free cash flow rose another £100m to £800m, supporting a forward dividend yield of 3.6% and leaving room for share buybacks. Yet it wasn’t enough. Net debt climbed again to £15.6bn. Forecast sales growth of 3%-5% across 2026 underwhelmed. I’m also concerned about its HIV medicines, as lucrative patents begin to expire in the next two or three years.

Drug companies operate under relentless pressure. Treatments take years to develop, trials can fail at the final hurdle, and there are political risks too. US tariffs remain a concern, although GSK has expanded manufacturing investment in America to reduce exposure there.

Even so, I still think the shares look good value with a price-to-earnings ratio of 11.15. I believe GSK deserves a place in a long-term portfolio and remains well worth considering today. I’ll be keeping a close eye on that pipeline though.

Should you invest £5,000 in GSK right now?

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Harvey Jones owns shares in GSK.