• Explain to interested patients that drug-drug interactions can adversely affect how a medication performs.
• Note that this study found that smoking tobacco or marijuana lowered blood levels of a major anti-HIV drug — but only in patients who abused another drug.
• Note that this study was published as an abstract and presented at a conference. These data and conclusions should be considered to be preliminary until published in a peer-reviewed journal.

SAN FRANCISCO — Smoking and substance abuse don’t mix for HIV patients under treatment with atazanavir (Reyataz), a researcher said here.

Whether it’s tobacco or marijuana, combining the smoke with another substance is likely to lower blood levels of the drug, according to Fatai Fehintola, MD, currently at the University of Ibadan, in Ibadan, Nigeria.

The danger is that patients could find the HIV medication less effective than otherwise, Fehintola said at the annual Interscience Conference on Antimicrobial Agents and Chemotherapy meeting.

“At face value, there could be a tendency for breakthrough of the virus,” said Fehintola, who — with colleagues — carried out the study at the University of Buffalo in N.Y.

On the other hand, in a short, six-month study, there was no immediate effect on viral load in the affected patients, he said.

“Down the line, there may be something,” he said. “But in this short period, there was no direct effect.”

Use of anti-HIV drugs has a clear benefit for patients, he said, but “the need to maintain adequate blood concentrations of the various drugs remains a vital component of treatment.”

Fehintola and colleagues hypothesized that concentrations of atazanavir might be affected by substance abuse, so they compared 32 HIV-positive people with a substance-related disorder with 35 who did not have such a condition.

All had been on atazanavir-containing therapy for more than two years, he said.

At each of three clinic visits, the volunteers had blood tests for drug concentrations and for viral loads and CD4 cell counts.

The researchers found that 49% of the volunteers smoked tobacco, while 28% used alcohol, 10% cocaine, 18% marijuana, and 19% opioids. Nearly half — 43% — of patients abused multiple substances.

Analysis found that a large proportion of patients with substance-related disorders — those who smoked — had trough levels of atazanavir that were below the therapeutic range.

Specifically, tobacco smokers with a substance-related disorder had a trough level, on average, of 0.314 micrograms of atazanavir per milliliter of serum, compared with 0.957 for smokers who did not have such a disorder.

The difference was significant at P=0.009.

Similarly, marijuana smokers with a substance-related disorder had a trough level, on average, of 0.238 micrograms of atazanavir per milliliter of serum, compared with 0.593 for smokers without such a disorder.

The difference was significant at P=0.03.

On the other hand, there was no significant difference between the groups with and without a substance-related disorder when they were stratified by alcohol, cocaine, or opioid use, the researchers found.

There were also no differences in viral load or CD4 cell count between the groups with and without a substance-related disorder, they said.

The study “highlights the fact that things people do every day may have drug-drug interactions,” said Scott Hammer, MD, of Columbia University in New York. Hammer, one of the meeting’s program co-chairs, was not part of the research.

Clinicians have to be “on the alert,” he said, and patients need to check before using any substance they haven’t cleared with their doctors. “We can be fooled by drug interactions that we’re not prepared for,” Hammer said.

He added that substance-using populations “need to be taken care of with as much force and care as non-substance using populations,” he said.