An anti-fungal drug used in the treatment of a rare infection called mucormycosis, or "black fungus", is in short supply across states in India.
Amphotericin B, which is manufactured by many Indian firms, is also on sale on the black market.
There are many emergency appeals on social media for the drug as mucormycosis cases rise.
Doctors say the infection could be triggered by the use of steroids in severely-ill Covid patients.
Mucormycosis is caused by exposure to mucor mould which is commonly found in soil, plants, manure and decaying fruits and vegetables.
It affects the sinuses, the brain and the lungs and can be dangerous in diabetic or severely immunocompromised people, such as cancer patients or people with HIV/AIDS. Many patients arrive for treatment late, when they are already losing vision, and doctors have to surgically remove the eye to stop the infection from reaching the brain.
Last week, Maharashtra's health minister Rajesh Tope said there were 1,500 cases of the infection in the state, which is one of the worst affected in the second wave of Covid-19 in India.
As many as 52 people people have died due to mucormycosis in the state since the coronavirus outbreak started last year, a senior health department told PTI news agency last week.
Officials in Gujarat state said that close to 900 cases of "black fungus" had been reported in the past month.
The owner of a big pharmacy in Ghaziabad city in Uttar Pradesh state told the BBC that earlier the injection had been easily available but had become difficult to procure since demand shot up three weeks ago.
With a severe shortage of the drug across cities, there has been a flood of frantic SOS pleas on Twitter.
Urgently required:— Sindhuja (@Sindhuj33493004) May 18, 2021
Patient name: RavindraNaidu
Age: 62 yrs
Please help@JSPSriram @charan_tweetz @6eChaithu @kiranbs45 @HiHyderabad @gopal_karneedi pic.twitter.com/k68Qjk5pcO
Very Urgent Requirement .. Help Needed for my Friend in #Delhi Admitted in hospital Max Super speciality hospital. Need #AmphotericinB injections #Ambisome .. Pls RT and help.. @SonuSood @ynakg2 @VK84015570 @vamsikaka @vizagobelix— M a n u (@Manu_Tweetz) May 18, 2021
Contact Attendee Number : 9704116367 pic.twitter.com/z3pSA7uxBv
Doctors say amphotericin B or "ampho-B" is an anti-fungal intravenous injection which has to be administered every day for up to eight weeks to patients diagnosed with mucormycosis. There are two forms of the drug available: standard amphotericin B deoxycholate and liposomal amphotericin.
"We prefer the liposomal form since it is safer, more effective and has lesser side effects. The flip side being that it is more expensive," Dr Akshay Nair, a Mumbai-based eye surgeon, told the BBC.
Concerns over mucormycosis are putting extra financial pressure on some families. Paying for treatment can run into hundreds of thousands of rupees. And families pay a lot more if they have to buy the drug on the black market.