Schizophrenics who took two antipsychotic drugs showed no more improvement than those on a single drug, raising doubts about the benefit of using multiple medicines to treat the disease, a study said on Wednesday.

The study, published in the New England Journal of Medicine, tested 68 patients who responded poorly to clozapine, the generic version of Novartis AG’s Clozaril.

Half the group took clozapine along with Johnson & Johnson’s antipsychotic Risperdal. The other half were given clozapine with a placebo. Both groups showed the same degree of improvement during the 26-week trial, but there was a slight deterioration in memory among the ones taking Risperdal.

William Honer, chief author of the study, said the results offer no support for the idea of prescribing multiple drugs, at least from the same class, especially to people who got little help from clozapine.

Schizophrenia affects 1 percent of the population and can trigger delusions, paranoia and hallucinations. It is extremely difficult to treat, and one third of patients are not helped by established medicines.

As a result, doctors combine medicines in as many as half of all schizophrenia cases, Honer said.

“Using two antipsychotic drugs together is quite common,” he said in an interview. “We’ve seen patients taking up to five antipsychotic drugs at the same time.”

In a Journal editorial, John Davis of the University of Illinois at Chicago said the results should not be considered definitive.

He said two other studies, which were funded by the pharmaceutical industry, indicated patients did better when they took the two drugs, perhaps because higher doses of Risperdal were used than in the Honer trial, which was not industry-sponsored.

In addition, Davis said, the Honer trial may have focused on patients who were too sick for Risperdal to make a difference.

Honer said doctors should work to make individual drugs more effective, such as finding the best dose, making sure the patients are taking it and supplementing the drugs with psychosocial treatments.