What is the expected growth rate of overall operating expenses for Zoetis in the next year
re of a medium-term growth driver. But I think there is a number of spaces where you're going to see innovation at the livestock spot. So across vectors, immunotherapies and precision livestock farming.
I'll let Glenn take the second question on long-term margin expansion.
Glenn David -- Executive Vice President and Chief Financial Officer
Yeah, so in terms of the long-term margin expansion, I think there are a couple of factors to consider. So for 2021, we mentioned that the gross margin is relatively flat and talked about some of the drivers of that with some of the investments we're making in reference labs. Also the impact of the DRAXXIN LOE this year on gross margin. So as we move into 2022 and beyond, some of those impacts will be a little less and we would expect to see continued expansion in gross margin.
In terms of the overall operating expenses, beyond 2021, obviously there will be one year where T&E normalizes when things get back to normal from COVID. But beyond that impact, we would expect that we would continue to be able to grow our expenses at a pace below that of revenue.
So we'll probably continue to grow R&D in line with revenue, that may vary any given year based on the opportunities, but SG&A based on the infrastructure that we have globally established, we should be able to grow that somewhere between inflation and revenue. So we would expect to continue to see expansion there on an overall operating margin perspective.
Operator
Our next question will come from David Risinger with Morgan Stanley. Please go ahead.
David Risinger -- Morgan Stanley -- Analyst
Yes, thanks very much and let me add my congrats on another phenomenal year as well. So I have two questions. First, with respect to the monoclonal antibody approval delays, so it's for both cats and dogs, but it wasn't quite clear to me whether the FDA wants more clinical data or whether there are manufacturing issues because of manufacturing questions because I think Kristin you had mentioned manufacturing. So if you could just clarify on both of them what the FDA issues are whether they are clinical or manufacturing?
And then second, Zoetis' R&D has obviously been amazingly differentiated from competitors. Competitors struggle to bring blockbuster companion animal products to market, even including follow-ons to Zoetis' top growth drivers over several years. And so considering that, can you just help us understand the unique aspects of Zoetis' R&D and its ability to maintain separation from the competition? Thank you.
Kristin Peck -- Chief Executive Officer
Sure. Thanks, Dave. With regards to the monoclonal antibodies, Librela and Solensia, it really is just working through the regulatory process and the questions that they are asking and they're requiring inspection of sites. So at this point we have not been asked for any clinical data, but we're still in the regulatory process is what I would say. It is the first time doing this with the FDA, so it's just honestly a new process for both and understandably it's the first time they're looking at some of these types of products, so they have a number of questions.
So it really is just going through the regulatory review process and trying to manage new manufacturing inspections, which I do know that probably COVID is definitely affecting that a little bit, but we're just working through that. So at this point we have not been asked for any additional clinical data and we don't think there's any manufacturing issues at this point, we're just still working through the review process and what their expectations are.
With regard to Zoetis R&D, what I would say is I think it's a partnership between R&D, manufacturing and our commercial organization. It's taking those insight the commercial has of customer needs and partnering early on with R&D to develop products. I think the other thing we've done really well is partner with manufacturing to be able to scale those products, and be able to bring them to market.
We manufacture our own monoclonal antibodies as you know, we've got very strong manufact
re of a medium-term growth driver. But I think there is a number of spaces where you're going to see innovation at the livestock spot. So across vectors, immunotherapies and precision livestock farming.
I'll let Glenn take the second question on long-term margin expansion.
Glenn David -- Executive Vice President and Chief Financial Officer
Yeah, so in terms of the long-term margin expansion, I think there are a couple of factors to consider. So for 2021, we mentioned that the gross margin is relatively flat and talked about some of the drivers of that with some of the investments we're making in reference labs. Also the impact of the DRAXXIN LOE this year on gross margin. So as we move into 2022 and beyond, some of those impacts will be a little less and we would expect to see continued expansion in gross margin.
In terms of the overall operating expenses, beyond 2021, obviously there will be one year where T&E normalizes when things get back to normal from COVID. But beyond that impact, we would expect that we would continue to be able to grow our expenses at a pace below that of revenue.
So we'll probably continue to grow R&D in line with revenue, that may vary any given year based on the opportunities, but SG&A based on the infrastructure that we have globally established, we should be able to grow that somewhere between inflation and revenue. So we would expect to continue to see expansion there on an overall operating margin perspective.
Operator
Our next question will come from David Risinger with Morgan Stanley. Please go ahead.
David Risinger -- Morgan Stanley -- Analyst
Yes, thanks very much and let me add my congrats on another phenomenal year as well. So I have two questions. First, with respect to the monoclonal antibody approval delays, so it's for both cats and dogs, but it wasn't quite clear to me whether the FDA wants more clinical data or whether there are manufacturing issues because of manufacturing questions because I think Kristin you had mentioned manufacturing. So if you could just clarify on both of them what the FDA issues are whether they are clinical or manufacturing?
 And then second, Zoetis' R&D has obviously been amazingly differentiated from competitors. Competitors struggle to bring blockbuster companion animal products to market, even including follow-ons to Zoetis' top growth drivers over several years. And so considering that, can you just help us understand the unique aspects of Zoetis' R&D and its ability to maintain separation from the competition? Thank you.
Kristin Peck -- Chief Executive Officer
Sure. Thanks, Dave. With regards to the monoclonal antibodies, Librela and Solensia, it really is just working through the regulatory process and the questions that they are asking and they're requiring inspection of sites. So at this point we have not been asked for any clinical data, but we're still in the regulatory process is what I would say. It is the first time doing this with the FDA, so it's just honestly a new process for both and understandably it's the first time they're looking at some of these types of products, so they have a number of questions.
So it really is just going through the regulatory review process and trying to manage new manufacturing inspections, which I do know that probably COVID is definitely affecting that a little bit, but we're just working through that. So at this point we have not been asked for any additional clinical data and we don't think there's any manufacturing issues at this point, we're just still working through the review process and what their expectations are.
With regard to Zoetis R&D, what I would say is I think it's a partnership between R&D, manufacturing and our commercial organization. It's taking those insight the commercial has of customer needs and partnering early on with R&D to develop products. I think the other thing we've done really well is partner with manufacturing to be able to scale those products, and be able to bring them to market.
We manufacture our own monoclonal antibodies as you know, we've got very strong manufact