Selegiline: How It Works and What to Watch

Selegiline is used mainly in Parkinson disease, and it works differently from the treatments most people know about. Rather than supplying dopamine, it stops the dopamine already present from being broken down.

That mechanism makes it useful early, useful alongside other treatment later, and gives it two peculiarities that catch people out — one about sleep, and one about drug tests.

🧬 How it works

Dopamine is continuously broken down by enzymes after it has done its job. Two related enzymes handle this, and they are distributed differently.

One predominates in the brain and is responsible for most dopamine breakdown there. The other predominates in the gut and liver, where its main job is destroying tyramine, a substance present in aged and fermented foods.

Selegiline blocks the first and largely leaves the second alone, at ordinary doses. Dopamine therefore persists longer where it is wanted, and the food restrictions associated with older drugs of this family do not apply.

That selectivity is dose-dependent, which matters. Push the dose high enough and the second enzyme is affected too, at which point the dietary considerations return — which is why the dose is not something to increase independently.

🎯 Where it fits

Parkinson disease is treated in stages, and selegiline appears at two points.

Early, on its own. In mild disease it produces a modest improvement in symptoms and can postpone the point at which levodopa becomes necessary. The benefit is real and it is smaller than that of levodopa, which is the trade being made.

Later, alongside levodopa. As the disease progresses, each dose of levodopa lasts less well and people experience periods when it has worn off before the next is due. Selegiline slows the breakdown of what levodopa provides, smoothing those troughs and reducing the time spent switched off.

It is also used, at higher doses and in a patch form, as an antidepressant — a different application in which the selectivity described above is lost and dietary restrictions do apply.

❓ Does it slow the disease

This deserves a direct answer because it was hoped for over decades and is still sometimes implied.

An influential trial found that people taking selegiline needed levodopa later than those who did not, which was initially read as evidence that the drug protected nerve cells. Subsequent analysis suggested a simpler explanation: the drug was relieving symptoms, so the threshold for adding levodopa was reached later. The apparent protection was the symptomatic benefit in disguise.

No treatment currently available has been shown to slow the underlying progression of Parkinson disease. Everything in use, including this drug, treats symptoms. That is worth knowing, both to set expectations and because it is the claim most often overstated by anything sold outside conventional treatment.

😴 Sleep, and why the timing matters

Insomnia is the most frequent complaint, and it follows directly from the amphetamine-like metabolites described above.

Selegiline is taken in the morning, or split between morning and midday, and not in the evening. Someone taking it at night will frequently lie awake, conclude the medicine does not suit them, and stop — when moving the same dose to breakfast resolves it.

This matters more than it sounds in Parkinson disease, where sleep is frequently disturbed already by stiffness, difficulty turning in bed, night-time urination and vivid dreams. Adding a stimulating drug at bedtime compounds a problem that was there anyway.

🧪 The drug test problem

Worth its own section because the consequences are entirely practical.

Because the body converts selegiline into amphetamine-related compounds, a urine screen can return positive for amphetamine in someone whose only exposure is a prescribed tablet. This is a well-documented finding rather than an occasional anomaly.

It matters for anyone subject to workplace testing, roadside testing, testing in a treatment programme, or pre-employment screening. The situation is resolvable — confirmatory testing distinguishes the forms produced by selegiline from those associated with misuse — and resolving it after the fact is considerably harder than declaring it beforehand.

Anyone taking selegiline who may be tested should say so in advance and carry evidence of the prescription.

🔴 The combinations that matter

Alongside those, decongestants found in cold and flu remedies can produce a rise in blood pressure, particularly at higher doses of selegiline where the selectivity is reduced. It is another argument for reading the ingredients of anything bought over the counter.

🧀 The food question

The older drugs in this family are famous for requiring avoidance of aged cheese, cured meat, fermented products and certain wines, because blocking the gut enzyme allows tyramine to reach the circulation and raise blood pressure sharply.

At standard doses selegiline largely avoids this, because it spares that enzyme. That is its main practical advantage over its predecessors and it is why no dietary sheet is usually issued.

Two qualifications. The selectivity is relative rather than absolute, so a very large tyramine load remains inadvisable. And it depends on the dose — at higher doses, including those used for depression in patch form above the lowest strength, restrictions do apply and are provided.

⚠️ Side effects

Commonly: insomnia, nausea, dry mouth, dizziness, headache. Most improve with timing, taking it with food, or a slower start.

Light-headedness on standing is worth attention because Parkinson disease itself lowers blood pressure on standing and levodopa adds to it. The combination causes falls, and a fall in this population is a serious event.

When added to levodopa, two things can worsen: involuntary movements, since more dopamine is available, and confusion or hallucinations, particularly in older people and in more advanced disease. Both are usually managed by adjusting doses rather than stopping outright.

Hallucinations deserve specific mention. They are common in Parkinson disease from the disease itself and from every drug used to treat it, they usually begin as a sense of a presence or fleeting movement at the edge of vision, and they should be reported early rather than concealed — they are far easier to manage before they become distressing.

💊 Taking it

In the morning, or morning and midday. Not in the evening.

With food if it causes nausea.

Consistently, since Parkinson treatment works on steady levels and missed doses show.

Not stopped abruptly where it is part of an established regimen — sudden withdrawal of Parkinson medication can cause a marked deterioration.

With every prescriber informed, including dentists, and with a list carried.

🔄 The alternatives

Selegiline is one of several drugs acting on the same enzyme, and the differences are practical rather than dramatic.

A more recently developed agent in the same class is taken once daily and, importantly, is not metabolised into amphetamine-related compounds — so it does not cause the same insomnia and does not produce the drug-test problem. Where either of those is an issue, it is worth asking about.

Beyond this class sit levodopa, which remains the most effective treatment available, and the dopamine agonists, which carry their own significant consideration — a recognised association with compulsive gambling, shopping, eating and sexual behaviour that is frequently not anticipated and is reversible on adjusting treatment. Anyone on that group should be asked about it directly rather than left to volunteer it.

🩺 When to seek help

• High fever with muscle rigidity and confusion — emergency, particularly after any new medicine
• Agitation, sweating, shivering and tremor developing over hours after adding another drug
• Before accepting any injected painkiller — say that you take selegiline
• Falls, or light-headedness on standing
• New hallucinations or confusion — early, while they are still manageable
• Persistent insomnia — frequently solved by moving the dose earlier
• New or worsening involuntary movements
• Compulsive gambling, spending or eating, if taking a dopamine agonist alongside
• Before any drug test, declare the prescription in advance
• A marked deterioration after doses have been missed

Treatment is stocked in the anti-Parkinsonian category, including Eldepryl (selegiline), Sinemet (carbidopa with levodopa), Mirapex (pramipexole) and Trivastal (piribedil). These are adjusted by response over time rather than set once, and none should be started or stopped without a prescriber who has the whole picture.

❓ Frequently asked questions

📑 Sources and editorial

• Prescribing information for selegiline, oral and transdermal
• Literature on monoamine oxidase subtype selectivity and dose dependence
• Studies of selegiline metabolism to amphetamine derivatives and urine drug screen interference
• The DATATOP trial and subsequent reanalyses of symptomatic versus neuroprotective effect
• Guidance on monoamine oxidase inhibitors with pethidine and with serotonergic agents
• Movement disorder society guidance on the management of Parkinson disease
• Comparative data on rasagiline and selegiline, including metabolite profiles
• Literature on impulse control disorders with dopamine agonist therapy
• Evidence on hallucinations in Parkinson disease and their management
• Related reading: Parkinson disease treatment, how dopamine drugs work
• Related products: anti-Parkinsonian category
• RXshop Editorial Team — reviewed by James Carter, MD, Neurologist