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Is a Stocks and Shares ISA really worth the effort? Here’s what the numbers say…

Mark Hartley breaks down the financial advantages a Stocks and Shares ISA can offer through its generous tax benefits. But what are the risks?

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By now, most Britons are probably fed up hearing about a Stocks and Shares ISA. I don’t blame them — whenever a new tax season rolls around (5 April), it becomes a hot topic of conversation.

But like it or not, there’s a good reason, and it shouldn’t be ignored. No matter your financial situation, saving for retirement is a good idea — and an ISA can make a huge difference.

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Why? Let’s crunch some numbers.

Compounding cash

Imagine you dump £15,000 into a portfolio of high-yielding dividend stocks that achieve a 10% annual return. That’s a meaty £1,500 gain – but you could lose £300 (or more) of that to tax!

If you put it into an ISA and avoid tax? It compounds. And after 10 years you’re up to £40,000. Without the ISA? You’d have likely lost at least £7,000 of that to tax.

That’s not pennies, that’s real money. And the longer it compounds, the bigger it gets — exponentially.

Please note that tax treatment depends on the individual circumstances of each client and may be subject to change in future. The content in this article is provided for information purposes only. It is not intended to be, neither does it constitute, any form of tax advice. Readers are responsible for carrying out their own due diligence and for obtaining professional advice before making any investment decisions.

So what’s the catch?

Well, we ARE talking about the stock market. It’s unpredictable and for novice investors, all the numbers and graphs can be intimidating. But in reality, it’s little more than eBay for trading shares.

Still, money can be lost. That 10% return one year can just as easily be a 10% loss the next. So how can we trust a company to keep turning a profit?

For the same reasons we trust a dentist in a town where everyone has healthy teeth. A much needed service, consistent demand, repeat customers, and a reputation for quality.

So which shares match that criteria?

Identifying quality

Becoming truly familiar with the ins-and-outs of a business can take years. Fortunately, fundamental analysis gives us a nice snapshot of what’s going on behind the scenes.

For example, few people are familiar with the biopharma outfit Oxford BioMedica (LSE: OXB). Yet the stock is up 113% in the past year — more than almost any other healthcare stock on the FTSE 100 or FTSE 250.

The company isn’t even profitable yet, posting a £30m loss in latest results. Yet revenue is up 31% year on year and the average broker expects a 47% price gain in the coming year. Case in point: analysts at Jefferies recently raised its price target to £8.27, reiterating a Buy rating.

So why would anyone be that optimistic about a £680m experimental gene therapy company?

On Monday (13 April, 2026), Oxford Biomedia launched a fast-track offering for its adeno-associated viral (AAV) and lentiviral vector platforms. This wasn’t just a promo stunt — it was a response to growing client demand and will help cut manufacturing timelines in half.

And that’s just one development as part of a wider and aggressive expansion into the US, one that promises to fuel momentum well into 2028.

But that doesn’t guarantee anything. If its US expansion doesn’t go as planned and financing runs dry, it could run into trouble — and lose investor confidence.

The bottom line

Investing in shares can deliver greater returns than a standard savings account, and using an ISA can maximise that growth.

But whether investing in FTSE 100 blue chips or emerging small-caps, there’s always risk. And when it comes to unprofitable companies like Oxford BioMedica, that risk is amplified. Still, with impressive developments and strong demand, it’s one worth considering, in my book.

Mark Hartley has positions in OXB. The Motley Fool UK has no position in any of the shares mentioned. Views expressed on the companies mentioned in this article are those of the writer and therefore may differ from the official recommendations we make in our subscription services such as Share Advisor, Hidden Winners and Pro. Here at The Motley Fool we believe that considering a diverse range of insights makes us better investors.

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