Theresa May has announced that homeopathic medicine will receive a specific exemption from the government's new law banning psychoactive substances, despite a shortage of scientific evidence that homeopathic medicine actually has any psychoactive effect.

The home secretary has written to the Advisory Council on the Misuse of Drugs to confirm that the government will use parliamentary time to amend draft legislation in order to specifically exempt all homeopathic treatments from the forthcoming psychoactive substances bill, which is designed to act as a blanket ban on any substances that have an effect on the brain.

Homeopathic medicine involves ingesting highly diluted substances and is described by NHS Choices as a treatment that performs "no better than placebos" and is based on principles that are "scientifically implausible".

Despite this, May said the government will use parliamentary time to ensure the forthcoming ban on mind-altering substances does not inadvertently criminalise homeopathy providers.

May wrote: "We are now proposing an amendment that will exempt all homeopathic and herbal products from the Bill. These will continue to be regulated by medicines legislation."

The letter was copied to health secretary Jeremy Hunt and policing minister Mike Penning, both of whom signed a 2007 motion in parliament backing the provision of homeopathic treatment on the NHS.

In the letter May also rejected concerns from the drugs advisory council that her proposal to ban "any substance intended for human consumption that is capable of producing a psychoactive effect" is too broad a definition that will inadvertently criminalise the use of substances that do not harm society.

The home secretary said she is concerned that a tighter definition of a banned substance would reduce the number of substances banned by the new law: "The [council] also suggested narrowing the definition of a psychoactive substance to focus on substances with a pharmacologically similar response and comparable public health threat to that of controlled drugs."

"I believe this approach would lessen the number of substances caught by the Bill, limiting the number of psychoactive substances caught to those which produce pharmacologically similar responses to substances controlled by the Misuse of Drugs Act 1971. ... I wish to avoid such uncertainty."

May said she is concerned that if the government only banned substances that pose a "comparable public health threat" to existing illegal drugs then fewer people would be prosecuted, she said, because the state "would have to show once again that the substances display similarities" to existing illegal substances.

She admitted that prosecutors would struggle to prove there is comparable public health threat, because there is rarely enough public scientific data on the level of harm caused by new psychoactive substances.

As a result, she would prefer a blanket ban to avoid the risk of any substances slipping through the net: "The avoidance of criteria for manufacturers of these dangerous substances which allow them to try to circumvent (or indeed incentivise them to), the provisions of the Bill is paramount."

May also confirmed that she will be unable to accommodate many of the demands of her advisory council, which consists of independent scientific advisers, on the drugs legislation.

"I appreciate you may be disappointed that I have been unable to accept all your recommendations," she told them in the letter.

Last week the House of Commons home affairs select committee said there were "serious concerns" about the government's rush to legislate on the issue and criticised May for ignoring the concerns of the advisory council while pushing to ban substances such as poppers that do not cause major harm to society.

"Legislating on this issue is the right thing to do, however, doing so at speed without any consultation may be counterproductive," said committee chair Keith Vaz. "The concerns expressed have been dealt with in a piecemeal manner and there has been unsatisfactory communication with the Advisory Council, the very body that the Ministers should rely on for advice."

The bill has been criticised by proponents of a liberal drugs policy for having a broad definition of psychoactive substances that could inadvertently ban some innocuous substances while giving exemptions to alcohol and tobacco. Concerns have also been raised by the Catholic Church, which at one point feared the provision of incense in church buildings could be banned due to the effect the substance has on the brain. The Home Office has since confirmed that churches will be allowed to keep burning incense.