Teva, which won U.S. approval in January to make an inhaler similar to Advair, also launched a generic version of its own inhaler, AirDuo RespiClick.
AirDuo is not a true generic of Advair, but contains the same two active ingredients, fluticasone propionate and salmeterol. However, it delivers a lower dose of salmeterol and uses Teva's RespiClick inhaler rather than copying GSK's device.
"We imagine that there should be sufficient time window for AirDuo to be the sole competing product to Advair," Raymond James analyst Elliot Wilbur said in a client note.
Advair, which brought in 1.83 billion pounds ($2.35 billion) in 2016 sales, is also widely used to treat chronic obstructive pulmonary disease, while AirDuo is only approved for asthma and is not directly substitutable.
"Neither branded AirDuo, nor its authorized generic are therapeutically equivalent or substitutable for Advair," GSK's spokeswoman Sarah Alspach told Reuters.
Still, Teva's product is likely to grab some share of the asthma market. AirDuo could capture 25 percent of the market by 2018, Wilbur said.
However, the bigger threat to GSK will likely come from fully substitutable generic copies of Advair that are still awaiting approval.
GSK has been preparing for the loss of Advair's exclusivity for the past two years. But the potential launch of generics will still be a blow since the medicine is highly profitable and has sold more than $1 billion annually since 2001.
Teva's AirDuo will cost wholesalers or direct purchasers $285, while the generic version will cost $90, Michelle Larkin, a spokeswoman for the Israeli drugmaker, told Reuters.
The pricing of the branded drug was in line with that of its peers, Bernstein analyst Erica Kazlow said.
"Teva's willingness to use generic strategy for the product is encouraging as it relates to the company's willingness to look at a tough reality and take appropriate action," Kazlow said.
($1 = 0.7798 pounds)
Reporting by Divya Grover in Bengaluru; Editing by Saumyadeb Chakrabarty and Savio D'Souza