Is constant sniffing for about half an hour after coming back from the bathroom a sign of drug abuse?

My friend at work goes to the bathroom and when he comes back he sniffs oddly for about half an hour. It’s not like a runny nose kind of sniff but just short little puffs every couple of seconds. It only happens when he goes to the bathroom and my other co-workers have noticed it too. When we ask him if he needs a tissue or something for his sniffles he acts like he doesn’t know what we are talking about. Do you think he might be doing drugs? If so what drug would cause that behavior?

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