Are you allowed stimulants prescribed by the doctor?
Answer:
In the name of Allah, the most Beneficent, the most Merciful.
The definition of a stimulant is as follows
Stimulants (also referred to as psychostimulants) are psychoactive drugs which induce temporary improvements in either mental or physical functions or both. Examples of these kinds of effects may include enhanced alertness, wakefulness, and locomotion, among others. Due to their effects typically having an “up” quality to them, stimulants are also occasionally referred to as “uppers“. Depressants or “downers”, which decrease mental and/or physical function, are in stark contrast to stimulants and are considered to be their functional opposites.
With regards to stimulants if they are used as medicine and it improves ones health then it will be permissible to use. However if it intoxicates a person or used for intoxication then this will be impermissible as mentioned in the Hadith
Prophet 
said: “Every intoxicant is prohibited (Haram).” (Sahih al-Bukhari, No:5659, Abu Dawud No:3200)
Only Allah Knows Best
Written by Molana Akbarul Hoque
Checked and approved by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham