Women going through the menopause are facing a national shortage of hormone replacement therapy (HRT).
Many of the most commonly prescribed forms of HRT, which helps deal with the symptoms of the menopause, are out of stock in some pharmacies.
The Department for Health and Social Care said it was aware of "ongoing supply issues" due to "manufacturing delays".
A spokesperson said alternative HRT products were available.
Affected patients should discuss alternatives with their doctor, they said, adding that the government was working closely with suppliers to "maintain the overall flow of medicines to patients".
Several women have told to the BBC how they have been affected, with one saying she feels "absolutely devastated" and another suffering "considerable discomfort and distress".
Chair of the Royal College of GPs Professor Helen Stokes-Lampard said the reasons for the delays were unclear.
"We know that there are the generic phrases like 'supply issues' and 'manufacturing problems,'" she said.
"But because it's commercially sensitive... nobody will be honest with the public and the NHS. So it's frustrating."
Speaking to BBC Radio 4's PM programme, she said: "It's really complicated and as a GP myself it's an intensely frustrating problem.
"Occasionally over the years we've had shortages of drugs and we have to deal with that. But the last six months in particular have been spectacularly difficult for HRT."
She urged women on HRT to start thinking about their getting their repeat prescriptions earlier than usual.
She said if patients found their pharmacies were out of stock of their prescription, they should ask the pharmacist what similar treatments were available and feed that back to their GP surgery.
'I haven't been me'
One woman, Rachel, said her HRT went out of stock at the end of last year and she has struggled to find something as effective.
"In the meantime I have been tearful, angry, overwhelmed at work, tired and have struggled with some physical symptoms. I simply haven't been me.
"I know none of it is life-threatening, but the impact on life, work and family is greater than I'd ever expected."
Another woman, Judie, said she has had unwanted side effects after being forced to try alternative HRT, while Joan said: "[I am] absolutely devastated.
"I cannot function without my medication. I lose concentration, confidence. I am sleep deprived and constantly anxious."
Meanwhile, Ava, a trans woman who is taking HRT, said: "As I'm sure you can imagine, the realisation that my ability to continue on the course I've committed myself to for the last three years is dependent on such a fragile supply chain is not a happy one and leaves me feeling rather anxious whenever my supply gets low."
Prof Stokes-Lampard said it was not clear how long the shortages would continue. Some products were experiencing a "temporary blip", she said, but for others it would be into early next year before the supply problems were fixed.
HRT is the most common treatment for menopausal symptoms such as hot flushes and night sweats, and involves taking oestrogen to replace the decline in the body's own levels, the NHS website says.
It is available as tablets, skin patches, a gel to rub into the skin or implants.
Around a million women in the UK use treatment for menopausal symptoms, according to the National Institute for Health and Care Excellence.