When taken orally, methadone usually takes effect within one to two hours. It produces a general sense of wellbeing and euphoria, sometime described as a ‘Ready Brek glow’. It isolates the user from feelings of anxiety and pain, both physical and emotional. Those with little or no experience of opiates may initially vomit or simply feel nauseous.

The ‘high’ of methadone differs from that of heroin – a steady ‘plateau’ with little of the euphoric peaks and gross sedation associated with smack. Methadone, like all opiates, causes contraction of the pupils (‘pinning’), which can be quite noticeable at higher doses. Some, especially new or inexperienced users, may experience a not-unpleasant itching sensation, particularly affecting the nose and legs. It is also highly constipating, and many addicts on long-term methadone maintenance are also prescribed laxatives.

Methadone may produce what is clinically known as ‘paradoxical stimulation’: users may feel that they ‘speed’ while on the drug. Whilst the drug has no genuine stimulant properties, it can provide resistance to physical fatigue and the impression of increased stamina as a by-product of its analgesic effect. Mild overdose results in nausea, vomiting, clamminess, sweating and a general feeling of weakness. At larger doses, breathing becomes shallow, pulse and blood pressure can become dangerously low and, in the most serious cases, coma and death may result.

Methadone has a residual effect, accumulating with repeat doses. It is this effect which makes it so useful in treating addiction, and also dangerous in terms of addiction potential. Someone who takes a dose of methadone on one particular day may still be at least partly affected by the drug 24 hours later.