Buying more of what you already have is expensive – and can be frustrating. Ask GlaxoSmithKline, which is lifting its ownership of subsidiaries in key emerging markets. It paid $1bn this year to take its stake in GSK Consumer Healthcare in India from 43 to 75 per cent. Yet a similar move in Nigeria, for a much smaller sum, failed after investors demanded a higher price. It is a tricky manoeuvre to pull off.

That has not deterred GSK. On Monday it said it would lift its stake in GSK Pharmaceutical, its India drug manufacturing unit, from 50 per cent to 75 per cent. It has launched a voluntary open offer for the shares at Rs3,100 a share, valuing the stake at about $1bn. The offer sent shares in the target up 20 per cent, to an all-time high of nearly Rs3,000.

A counter-offer is improbable, so shareholders in GSK Pharma should accept the cash. Not only is GSK offering a premium of 26 per cent to the undisturbed price, but an illiquid stock threatens to become even more so. (GSK is pledging to honour Indian stock market rules to keep a 25 per cent free float.)