“We should be planning in a smart way to make these switches when they become possible,” said Mark Harrington, executive director of the Treatment Action Group, which works globally to help people get medicines for H.I.V., tuberculosis and hepatitis C. “If this were a rational world, this would set in play a dynamic series of negotiations between buyers and manufacturers, trying to bid around for the lowest price,” he said.

But what an irrational world it is! What’s new about Mylan’s Cimduo is that it combines two medicines available in generic form: tenofovir and lamivudine. If Cimduo is interchangeable with Truvada, then so are these drugs. The difference? Cimduo is two pills (taken together) instead of one. Oh, and the price — together the two generics add up to $1,635 per year.

Someone might say: Let’s prescribe both generics and we’ll save $18,000 off the cost of Truvada!

The same is true for Mylan’s three-drug combos. The cost of the three generic drugs in Symfi and Symfi Lo is $12,000. Prescribing those individual drugs instead of Atripla would mean taking three pills instead of one — and saving $20,000.

No one has.

This isn’t an accident. H.I.V. drug pricing is a mess because drug companies have so much sway over government policy. For example: Medicare is not allowed to negotiate with drug companies on price — although 92 percent of Americans think it should. If Medicare could get the same prices that, say, the Department of Veterans Affairs gets, it would save nearly $3 billion a year on just the 20 most-prescribed drugs, according to a study by Senate Democrats.

Pharmaceutical companies have built a system in which no one in a position to limit costs wants to do so.

The federal panel on antiretroviral guidelines now does include references to drug costs, said Dr. Rochelle Walensky, chief of infectious diseases at Massachusetts General Hospital and a guidelines panel member. “But the first-line regimens recommended still favor the absolute best — and often most expensive — drugs available. It’s ‘if this were my dad, what would I want him to have?’” she said.

Insurers should care about price. But it’s hard for them to refuse to cover a recommended drug, and they can pass their costs along in the form of higher premiums.