In the past, studies have shown that the risk of a child developing asthma increases when A) the mother uses antibiotics during pregnancy, or B) the child uses antibiotics early in life. This lead to a widespread belief that antibiotics can cause asthma.

But a group of researchers from the Karolinska Institutet in Sweden felt that previous studies had not “sufficiently examined shared risk factors within families that could increase asthma risks, including genetics, home environment and lifestyle.” So from 2006 to 2010, these researchers examined nearly half a million Swedish children and used a sibling control analysis to see if there really was a causal link between asthma and antibiotics.

For example, previous studies concluded that when a mom uses antibiotics during pregnancy, the child has a 28% higher chance of developing asthma. But when researches included other risk factors such as the child’s genetics and environment by comparing multiple children within the same family, the link between asthma and antibiotics disappeared.

The researchers believe the correlation previous studies found between asthma and antibiotics could be explained by other factors. For example, a childhood respiratory infection that gets treated by antibiotics might be a misdiagnosed case of asthma. Plus, having a respiratory infection at all could lead to an increased asthma risk, whether it’s treated by antibiotics or not.

Researcher Professor Catarine Almqvist Malmros points out that even though there study showed “no causal link between antibiotics treatment and childhood asthma” that it’s still “important to use antibiotics very carefully, considering the threat of antibiotic resistance.”