U.S. regulators told Abbott Laboratories Inc. on Thursday to stop using misleading price claims that were circulated about its AIDS pill Norvir as the company defended a 400 percent price hike on the drug.

Information on a Web site and in other promotional materials wrongly portrayed the drug, Norvir, as the lowest-priced drug of its kind, the FDA said in a warning letter to the company.

Abbott also failed to completely disclose the drug’s risks, the FDA said.

Abbott’s material “misleadingly claims that Norvir has the lowest daily cost of all antiretroviral drugs, and minimizes the risks of Norvir,” the FDA warning letter said.

Regulators told Abbott to immediately stop the promotions and devise a plan to circulate accurate information to the audiences that received misleading messages.

Abbott did not intend to give false impressions about the drug’s use and will modify the materials, company spokeswoman Melissa Brotz said.

Norvir, known generically as ritonavir, is a protease inhibitor that helps suppress the HIV virus that causes AIDS. The drug is unique in its class because it can make other HIV medicines more effective.

Abbott last December raised the price of a 100-milligram Norvir capsule to $8.57 from $1.71. The company said the price hike was necessary to reflect the drug’s true value and to help fund future research. AIDS activists and some physicians called the higher price unreasonable.

In a chart circulated after the price hike, Abbott compared a 100-milligram-per-day dose to the costs of other AIDS medicines that ranged from $9.84 to $32 per day.

The FDA said the comparison was misleading because the agency has deemed Norvir safe and effective only at doses between 300 milligrams and 600 milligrams twice daily.

The materials also implied Norvir could be used alone, but it is approved only for use with other AIDS drugs, the FDA said.

The chart “raises significant public health and safety concerns because of ... the potential adverse impact these false and misleading messages may have on the HIV community by promoting a subtherapeutic dose,” the FDA said.

The FDA also said Abbott’s presentation of safety risks was incomplete, in part because the company failed to mention two drugs that could cause life-threatening reactions if taken with Norvir.

Abbott developed the chart in response to concerns about the price hike and did not intend to promote Norvir as a stand-alone drug at an ineffective dose, Brotz said.

“Abbott will modify the materials as necessary in accordance with FDA guidance,” she said.