Does the one who smokes marijuana come under the same ruling as the one who drinks alcohol?

Firstly: 

Before discussing the ruling on marijuana, it is essential to find out what it really is, and the effects that are caused by this substance.

Marijuana is one of the derivatives of the cannabis plant (Cannabis indica). This is a plant that has an intoxicating effect, and it is very common in Arab countries where it is known by a number of names (such as “bangue” in Egypt; bangue is something other than hasheeh), or by the Western names for it, such as marijuana.

The unprocessed form of the drug is composed of the dried mature flowers and leaves of the plant. 

The processed product is known as hashish, which is basically composed of the glandular trichomes that are collected from the same plants. The main active substance in cannabis is the organic chemical compound Delta nine tetrahydrocannabinol, which is known as THC. 

Hashish is a hallucinogenic substance when taken in large doses. Smoking hashish is the most common method of consumption and has the greatest effect on the central nervous system, because the active substance quickly reaches the bloodstream from the lungs, and from there it reaches the brain, making the individual feel relaxed, drowsy, happy, euphoric and joyful; it makes him feel very weak, unable to concentrate or focus. It also affects the short-term and working memory. The individual may also lose balance and experience impaired motor skills, increased heart rate, increased impulsivity, a fall in blood pressure, and dryness of the mouth and throat.

End quote from Wikipedia, the Free Encyclopedia 

Secondly: 

Once the nature of marijuana becomes clear, we will realise that it is indeed a kind of khamr, and all the rulings on khamr are applicable to it. Shar‘i rulings cannot be determined on the basis of one’s reasoning or ideas; rather they are to be based on shar‘i texts and the rulings of Allah and His Messenger on the matter. The Prophet (blessings and peace of Allah be upon him) called all intoxicants khamr and applied all the relevant rulings to them: 

Muslim narrated in his Saheeh (2003) that Ibn ‘Umar (may Allah be pleased with him) said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “Every intoxicant is khamr and every intoxicant is haraam. Whoever drinks khamr in this world and dies when he is addicted to it and has not repented, will not drink it in the Hereafter.” 

If marijuana is an intoxicating substance, as explained above, and what it is and what it causes is well-known and well-established, then it is undoubtedly khamr and is subject to the same rulings as khamr in this world and the hereafter. 

It does not matter if something only causes intoxication when consumed in large amounts, but small amounts do not cause intoxication, because the Prophet (blessings and peace of Allah be upon him) said: “Whatever causes intoxication in large amounts, a small amount of it is haraam.” 

Narrated by an-Nasaa’i (5607) and others; classed as saheeh by al-Albaani. 

Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) discussed this matter at length, explaining the rulings on what you have asked about. He discussed this substance (marijuana or hashish) and stated that all the rulings on khamr are applicable to it. He (may Allah have mercy on him) said:

As for the accursed intoxicant hashish, it is the same as other intoxicating substances, and what intoxicates of it is haraam according to scholarly consensus. In fact anything that causes a person to become unconscious is prohibited to eat, even if it does not cause intoxication, such as banj (Hyoscyamus niger, commonly known as henbane or stinking nightshade, a plant with psychoactive properties that was historically used as an anaesthetic and for other purposes). The hadd punishment is required for consuming intoxicating substances, and a disciplinary punishment (ta‘zeer) may be imposed in other cases, where the substance consumed does not intoxicate but it renders one unconscious. 

Consuming small amounts of intoxicant hashish is haraam according to the majority of scholars, as is also the case with regard to consuming small amounts of any other intoxicant. 

The words of the Prophet (blessings and peace of Allah be upon him), “Every intoxicant is khamr and every khamr is haraam” include everything that causes intoxication, and it makes no difference whether the intoxicant is eaten or drunk, or whether it is solid or liquid. If the hashish is turned into liquid and drunk, it is still haraam. 

Our Prophet (blessings and peace of Allah be upon him) was sent with concise speech, so if he spoke a comprehensive word that was general in meaning, a lot of words and meanings may come under that category, whether they are the exact same things that existed in his time and in the place where he lived, or otherwise. When he said “Every intoxicant is haraam,” that included what existed in Madinah of intoxicating drinks made from dates and so on, and it also included what existed in Yemen of intoxicating drinks made from wheat, barley, honey and other things. It also includes what was introduced later on of alcoholic drinks made from mare’s milk, which was common among the Turks and others. None of the scholars differentiated between intoxicants made from mare’s milk or intoxicants made from wheat and barley, even though one of them existed and was known at the time of the Prophet (blessings and peace of Allah be upon him), and the other was not known at that time, because no one in the Arab land made intoxicating  drinks from mare’s milk. 

The first report we have of hashish appearing among the Muslims was at the end of the sixth century AH and beginning of the seventh century, when the Tatar state emerged, at the time when Genghis Khan attacked the Muslims. At that time, people began to commit openly acts that Allah and His Messenger had forbidden, and because of these sins Allah caused the enemy to prevail over them. This accursed hashish was one of the greatest of evils, for it is worse than drinking intoxicants in some ways, and intoxicant drinks are worse than it in other ways. In addition to causing the one who consumes it to become intoxicated to the extent that he is completely dazed and stupefied, hashish also causes a person to develop an effeminate attitude, become a cuckold (who does not care about impropriety on the part of female family members), and to become disturbed. It makes one eat a great deal and may lead to insanity. Many people have become insane because of consuming it. 

Some people say that it alters the mind, so it does not cause intoxication like banj (henbane), but that is not true. Rather it gives a sense of pleasure and euphoria like alcohol, and this is what prompts people to consume it. A little of it leads to more, like intoxicating drinks, and if a person becomes used to it, it is more difficult to wean oneself from it than is the case with alcohol. So in some ways the harm it causes is greater than that caused by alcohol. 

Hence the fuqaha’ said that the hadd punishment must be given for it, as in the case of alcohol, but they differed as to whether it is najis (impure). … 

If a person becomes intoxicated as a result of drinking alcohol or consuming intoxicating hashish, it is not permissible for him to come near the mosque until he sobers up, and his prayer is not valid until he knows what he is saying. He must wash his mouth, hands and garments in either case (i.e., whether he has drunk alcohol or consumed hashish), and prayer is still obligatory for him, although it will not be accepted from him for forty days, unless he repents, as the Prophet (blessings and peace of Allah be upon him) said: “Whoever drinks alcohol, no prayer will be accepted from him for forty days, then if he repents, Allah will accept his repentance. If he drinks it again, no prayer will be accepted from him for forty days, then if he repents Allah will accept his repentance. Then if he drinks it again, it will be incumbent upon Allah to give him to drink of the mud of al-khabaal.” It was said: What is the mud of al-khabaal? He said: ‘The juice of the people of Hell – or the sweat of the people of Hell.” 

As for the view of those who say that there is no verse or hadith about marijuana, this stems from ignorance, for the Qur’an and hadith contain comprehensive words which state general principles and holistic issues, and cover everything that comes under that heading. Everything that is included under that heading is included in the Qur’an and hadith in general terms, but it is not possible to mention every single thing by its specific name. 

End quote from Majmoo‘ Fataawa Shaykh al-Islam (34/204-207) 

What you must do is give up this serious problem and be strong in adhering to the command of Allah. Do not compromise with regard to your whims and desires; preserve your religious commitment and protect your prayer from consuming khamr, lest khamr (intoxicating substance) lead to ruin in your religious and worldly affairs. If the matter requires medical rehabilitation, then you must do that immediately and seek help thereby to give up this problem that you are faced with. 

We ask Allah to open your heart to that which He loves and is pleased with of words and deeds, and to bless you by enabling you to repent sincerely. 

And Allah knows best.